Introduction

Recently the conodont species Idiognathodus simulator (Ellison, 1941) was chosen to be the marker species for the base of the global Gzhelian stage (Heckel et al. 2008). Because of this decision, the I. simulator group and closely related species will be used for the correlation of the Kasimovian-Gzhelian boundary around the world. Since the original description of I. simulator by Ellison (1941) from Midcontinent North America, additional species bearing an eccentric groove like I. simulator have been described and named from different regions (Donets Basin, Kozitskaya et al. 1978; south China, Wang and Qi 2003; southern Urals, Chernykh 2005, 2012). This study presents a new geometric morphometric analysis that provides a clearer understanding of morphological variation among P₁ elements of the I. simulator group recovered from the Heebner Shale of the Oread cyclothem, the oldest Gzhelian cyclothem in the North American Midcontinent region. This new information permits a taxonomic revision that incorporates species concepts from all geographic regions and provides a more precise description of the diagnostic characteristics of I. simulator that will facilitate more accurate and precise correlation of the base of the Gzhelian Stage.

Institutional abbreviations.—SUI, University of Iowa Paleontology Repository, Iowa City, USA.

Other abbreviations.—CVA, canonical variate analysis; PCA, principal component analysis; SSCP, square roots and cross products; TT, morphometric identification number.

Historical background

Ellison (1941) erected Streptognathodus simulator and Streptognathodus eccentricus based on the possession of an eccentric groove that runs down the oral surface of the platform, a feature that distinguished them from other species of Streptognathodus, in which the groove, or trough, is medial in position. The feature used to differentiate these two species from each other was that S. simulator possessed nodes on only the caudal side of the P₁ element, and S. eccentricus possessed nodes on both sides of the P₁ element. Streptognathodus simulator was described from the early Virgilian Heebner Shale of the Oread Limestone of Midcontinent North America, and S. eccentricus was described from the early Missourian Hushpuckney Shale Member of the Swope Limestone (Ellison 1941). Ellison (1941) reported that S. eccentricus ranged through most of the Missourian into the Virgilian, and that S. simulator ranged from the late Missourian into the Early Permian. For approximately the next five decades, Ellison's (1941) species concepts were followed by most workers. Kozitskaya et al. (1978) added a third species to the I. simulator group from Upper Pennsylvanian strata in the Donets Basin, Ukraine, describing the species S. luganicus, which possesses the eccentric groove, but lacks nodes completely and has platform margins that extend ventrally parallel to the carina.

Barrick and Boardman (1989) proposed that Streptognathodus eccentricus did not belong to the genus Streptognathodus, but was actually a distinct early Mis sourian form of Idiognathodus that was older than the first appearance of the genus Streptognathodus in the middle Missourian. This argument was based on their restriction of Streptognathodus to a clade of species that appeared in the middle Missourian, and the exclusion of other similar, but unrelated grooved species from the genus (Rosscoe and Barrick 2009). This was in contrast to the general practice, following Ellison (1941), of including all idiognathodids with any groove on the oral surface in Streptognathodus and those without a groove to Idiognathodus. Barrick and Boardman (1989) removed the species S. simulator from Streptognathodus, and assigned it to Idiognathodus on the basis that it is a member of a unique late Missourian and early Virgilian lineage of Idiognathodus unrelated to Streptognathodus, characterized by an eccentric groove on the oral platform surface. Barrick and Boardman (1989) reported that I. simulator ranged from the Missourian Eudora Shale to the Virgilian Heebner Shale and noted that specimens of I. simulator from the Eudora Shale could be differentiated from specimens from the Heebner Shale. Subsequent work in North America (e.g., Boardman and Heckel 1989; Heckel 1989, 2002; Ritter 1995) generally followed these reassignments. In their summary of Midcontinent conodont biostratigraphy, Barrick et al. (2004) modified the nomenclature to distinguish between the older and younger examples of I. simulator, using I. aff. simulator for the Eudora Shale specimens and I. simulator (sensu stricto) for the Heebner Shale specimens.

Around this time workers from other regions also began to describe faunas containing Idiognathodus simulator-like specimens. Wang and Qi (2003) proposed the species Streptognathodus luosuensis to include specimens that have a deflected carina. This species was described from the Nashui (Naqing) section in Guizhou Province, South China. Wang and Qi (2003) recorded S. luosuensis as being present only in the S. simulator Conodont Zone.

Chernykh (2005) added three new species to the Idiognathodus simulator group based on late Kasimovian and early Gzhelian specimens from the southern Urals. Specimens with a short, outward flaring rostral adcarinal ridge were designated as S. auritus, and specimens with a straight platform shape as S. praenuntius. For S. praenuntius, the groove was described as medial and less developed and the platform edges were described as having high ridges and ventral extensions shorter than those typical of I. eudoraensis, but longer than I. simulator. Streptognathodus sinistrum was distinguished from I. simulator by a slender platform, a high caudal ridge, and a deep trough separating the caudal ridge from the rest of the platform. Chernykh (2005) illustrated only dextral elements of S. auritus and S. praenuntius and sinistral elements of S. sinistrum. Additional discussion about these species from the Urals can be found in Chernykh et al. (2006) and Davydov et al. (2008).

Barrick et al. (2008) eventually named the older North American species, Idiognathodus aff. simulator, as I. eudoraensis and referred to I. simulator (sensu stricto) as simply I. simulator. Barrick et al. (2008) described I. simulator as an asymmetrical pair of P₁ elements. The sinistral element platforms are curved, have a well-developed eccentric groove, and short adcarinal ridges that occasionally fuse with nodes and lack ventral extensions. The dextral elements are relatively straight, arrowhead-shaped, have a less developed eccentric groove, and a flatter oral surface than the sinistral elements, and both adcarinal ridges lack ventral extensions. Barrick et al. (2008) based the new species I. eudoraensis on asymmetrical pairs of P₁ elements that occur in the Eudora Shale of Midcontinent North America, which they distinguished from the I. simulator that occurs in the younger Heebner Shale. The sinistral elements of I. eudoraensis are relatively straight, have a slightly developed eccentric groove, and long adcarinal ridges that form ventral extensions parallel to the carina (Barrick et al. 2008). The dextral elements of I. eudoraensis are similar to the dextral elements of I. simulator, but can be distinguished from each other by the ventral extension of the adcarinal ridges. Barrick et al. (2008) assigned S. luganicus to Idiognathodus and suggested that I. luganicus may appear in younger strata than I. simulator. Barrick et al. (2008) claimed that a deflected carina is not a species-level character, and stated that the specimens of S. luosuensis illustrated by Wang and Qi (2003) have ventral extensions parallel to the carina and appear similar in appearance to I. luganicus.

Chernykh (2012) later described two more species from the southern Urals. Streptognathodus gravis differs from S. auritus by having better developed lobes and no rostral ridge flaring. Streptognathodus postsimulator is a lobeless form with a strongly angled eccentric groove, and a broader platform than S. luganicus.

Geological setting

During Late Pennsylvanian time the Midcontinent sea of North America, and the Russian platform of Eurasia were separated by the large northern landmass of Protopangea and were connected only by a narrow northern seaway during periods of global sea-level highstands (Fig. 1A). During these periods of highstand, the Midcontinent sea of North America extended from the Appalachian highlands in the south, to the ancestral Rocky Mountains and, in places, connected to the Protopacific ocean “Panthalassa” to the west and northwest (Fig. 1B). Late Pennsylvanian deposition in the Midcontinent Basin was controlled by glacial-eustatic sea level rise and fall, which created repetitive and cyclic stratigraphic sequences “cyclothems” throughout the region (Heckel 1994, 1999). The Oread is a major cyclothem and contains in ascending order, the upper portion of the Snyderville Shale, the Leavenworth Limestone, the Heebner Shale, the Plattsmouth Limestone, and the lower part of the Heumader Shale of the Oread Formation of the Shawnee Group (Heckel 1999). The upper Snyderville Shale that occurs above a thin carbonate lag is interpreted as a transgressive fossiliferous shale and marks the beginning of the Oread cyclothem. The Leavenworth Limestone is the transgressive limestone of the cycle and is abruptly overlain by the Heebner Shale (Fig. 2A). The maximum transgressive surface of this megacycle occurs within the Heebner Shale. The Plattsmouth Limestone overlies the Heebner Shale and is the regressive limestone of the cyclothem. This limestone contains a diverse fauna of fusulinids, corals, bryozoans, crinoids and abundant phylloid algae that create wavy, algae-laminated bedding surfaces. The regression is interpreted to continue up section into the lower Heumader Shale, which represents marginal marine deposition and the end to the Oread cyclothem (Boardman 1999).

The Heebner Shale is 140 cm thick at the I-229 roadcut, 155 cm thick at Clinton Dam and 190 cm thick at the Old Sedan spillway (Fig. 2B). The lower 5 cm of each section is gray shale containing abundant brachiopods. The overlying black shale at the Sedan and Clinton sections is subdivided into two parts. The lower half is composed of dark fissile shale and the upper half is black fissile shale with intervals of phosphate concretions. These concretions form laterally continuous beds at Sedan, but are more randomly distributed at Clinton. A 3-cm-thick, laterally continuous and resistant bed of coarse phosphatic sand was observed only at Clinton. Ammodiscus, Planolites, and occasionally fish debris can be found in the black shale. The black shale at all three sections grades up into a gray shale that contains productid brachiopods, bryozoans, crinoids, and low-spired gastropods. The gray shale is most calcareous at the top and begins to form partially cemented beds just below the contact with the Plattsmouth Limestone.

Fig. 1.

Paleogeographic map of Laurentia (A) and the Midcontinent sea (B) during Late Pennsylvanian time. Terrestrial/marine environments represent regions dependent on glacio-eustatic and basin controls. Modern day state outlines for reference to sample locations: 1, Sedan spillway; 2, Clinton Dam; 3, interstate I-229 roadcut. A.W.A., Ancestral-Wichita-Amarillo mountains. A, based on Heckel (1999); B, modified from Algeo and Heckel (2008).

Material and methods

Conodont samples.—The Heebner Shale was sampled at five-centimeter intervals at three different locations within the Midcontinent region (Figs. 1B, 2B; Appendix 1). The northernmost of these outcrops is along interstate I-229 northwest of St. Joseph in Andrew County, Missouri. A central section is located at the Clinton Dam spillway just southwest of Lawrence in Douglass County, Kansas. The southernmost location is the old Sedan City Lake spillway in Chautauqua County, Kansas. (Fig. 1B; Appendix 1). All measurements in the following descriptions are taken upward from the contact between the base of the Heebner Shale and the top of the underlying Leavenworth Limestone.

Fig. 2.

A. Composite stratigraphic section of the lithologic members of the Oread Megacylothem and relative sea level curve; modified from Heckel (1999, 2013). B. Detailed measured stratigraphic sections of the Heebner Shale at each locality. Darkness of the shale intervals corresponds to the darkness of the shale in hand sample. See SOM 1 for exact locations.

Gray shale samples were dried and then submerged in kerosene for approximately 4 to 5 hours to aid in the defloculation of the clay grains. The kerosene was poured off, and the sample was submerged in hot water and allowed to sit overnight in order to thoroughly disaggregate the sample. Black shale samples were submerged in a 12% bleach solution (sodium hypochlorite) in order to break down the organic material. The solution was poured off and replaced every month until the shale began to lighten in color and break down into a fine residue. This process can take several months depending on the amount of organic material in the sample. Once the organics had been destroyed, the sample underwent the gray shale procedure. Limestone samples were crushed into small (1–3 cm diameter) pieces and dissolved in a 10% buffered formic acid solution to dissolve carbonate material. Samples were sieved to a small size (120 µ;m). When necessary, the residues were placed in heavy liquids for density separation.

Several thousand P₁ elements of Idiognathodus were collected from the Heebner Shale at the three outcrop localities. Idiognathodus specimens that possessed an eccentric groove were classified as belonging to the I. simulator group and specimens that lacked an eccentric groove were classified as belonging to either the I. tersus (Ellison 1941) or I. pictus groups (Chernykh 2005) (for details see SOM 1: tables 2, 4, 6; Supplementary Online Material available at  http://app.pan.pl/SOM/app61-Hogancamp_etal_SOM.pdf). Many P₁ elements of Idiognathodus were split in half along the eccentric groove, a breakage habit described by Von Bitter (1972). If a P₁ element was complete from the dorsal termination of the carina to the dorsal tip it was counted as the appropriate species. If the element met this criterion but breakage prevented conclusive identification of the specimen as one of a specific species then it was tabulated as an Idiognathodus or Streptognathodus P₁ element. Only every other split P₁ element was counted. Sinistral and dextral P₁ elements were counted separately to ascertain which dextral and sinistral morphotypes tend to occur together.

While picking specimens from residues, it was observed that all very small specimens lacked lobes and appeared nearly identical in morphology. Because juvenile specimens of many different animal species may appear identical early in their ontogenetic sequence, a size minimum had to be established before picking specimens for morphometric analysis. A minimum length of approximately 0.3 to 0.4 mm from the ventral tips of the adcarinal ridges to the dorsal tip was selected as the minimum size to evaluate lobe development. This means that the earliest ontogentic stage of these species is excluded from the subsequent analyses, but this omission allows for a more accurate comparison between adult morphologies.

Geometric morphometrics procedures.—Morphometrics is the quantitative study of shape and form. Traditional mor phometrics involves taking measurements of lengths, widths, angles, areas, ratios and other similar properties. These measurements are often made on particular features that the investigator might interpret as being homologous. It is important to note that any morphological information that is not measured is lost to the analysis. In contrast, geometric morphometrics focuses on the spatial relationships among anatomical “landmarks” (typically points marking the boundaries between abutting structures) and takes into account the geometric relationships of every landmark with every other landmark. More thorough discussions of the comparisons between traditional morphometrics and geometric morphometrics can be found in Rohlf (1990), Rohlf and Marcus (1993), and Adams et al. (2004). Various types of landmarks can be used, including ones that are not necessarily tied to truly homologous features. Because conodonts can have “plastic” features, dramatic ontogenetic change, a wide array of ornamentation, and are often discriminated on a basis of shape and features rather than size, geometric morphometrics is an ideal approach for characterizing their morphological features. Additionally, many conodont species have unique sets of shapes and characters that lend themselves toward the development of landmark-based analyses.

P₁ elements were photographed at an orientation such that the denticles of the carina were parallel to the angle of the camera. For the morphometric analysis of the Idiognathodus simulator group, 263 elements were included in the analyses. Photographs for morphometric analysis were all taken on the Nikon SMZ1500 using the same camera, file and magnification settings. JPEG files were converted to TPS format using the software TpsUtil (Rholf 2010a). Landmarks were digitized on the TPS files using the software TpsDig (Rholf 2010b). Multivariate analyses were performed and wireframe images were generated using the software MorphoJ (Klingenberg 2011) as described in the following sections (see SOM 2 for morphometric data).

Step 1. Landmark definitions: Bookstein (1991) defines three different categories of landmarks. Type 1 landmarks are interpreted as being truly homologous points between all specimens. Type 2 landmarks are points that can be defined and located on all specimens, but may or may not be truly homologous points. Type 2 landmarks are often used as helping points, or sliding landmarks along a boundary formed between Type 1 landmarks. Lastly, Type 3 landmarks are defined by measures of extremity such as end points of maximum length or width, and do not need to be placed along a homologous boundary. Further discussion and examples of the process of defining and digitizing landmarks can be found in Zelditch et al. (2004).

Fig. 3.

Transformation of a conodont P₁ element into morphometric coordinate points: 1, specimen is photographed; 2, coordinates are placed on the image using the software TspUtil; 3, wireframe outline is created by linking landmarks to each other; 4, image is removed to illustrate that all analyses beyond this point are performed on the landmarks only, and all other biological criteria are no longer considered. Open circles, location of Type 1 landmarks, and black ovals, location of Type 2 landmarks.

Four Type 1 landmarks and fourteen Type 2 landmarks were defined on the P₁ elements of the Idiognathodus simulator group using Bookstein's (1991) landmark definitions, resulting in a total of 18 landmarks for analysis (Figs. 3, 4). Dextral images were mirrored prior to landmark placement to allow specimens to be analyzed with natural chirality orientation effects removed. The numbers listed after the anatomical features in the following sentences reference the landmark numbers in Fig. 4, and the anatomical features defined in SOM 3. The Type 1 landmarks are the dorsal tip of the element (16), the dorsal termination of the carina (2), and the ventral terminations of the adcarinal ridges (1, 3). The Type 2 landmarks are the midpoints along the adcarinal ridges (17, 18), the rostral and caudal terminations of the most ventral and most dorsal transverse ridges (4–7, 12–15), and the rostral and caudal terminations of the transverse ridges nearest the point of maximum curvature along the caudal and rostral platform margins (8–11).

Step 2. Definitions of groups for morphometric analysis: Specimens from the Idiognathodus simulator group were assigned membership to a number of qualitative groups. Groups were chosen to focus on anatomical features that have been used in the previous species discrimination of Idiognathodus. These groups included categories defined by chirality, groove, lobes, geographic location, and a series of morphotype groups. Chirality membership was either sinistral or dextral. Lobe group membership was divided into four categories, 0 lobes, 1 lobe, 2 lobes, or a disconnected adcarinal ridge. Location group was one of the three stratigraphic sections, Clinton, Sedan, and I-229. Specimens with more than two truncated longitudinal transverse ridges were classified as grooved, and those with two or fewer truncated transverse ridges were classified as non-grooved. The term “incomplete” was used to designate specimens that had over half of their transverse ridges completed, but still were classified as grooved based on the two ridge criterion. In the morphometric analyses these “incomplete” specimens were included as “grooved” to maintain purity of the “grooveless” classifier.

Fig. 4.

Wireframe created by connecting landmarks chosen for the Idiognathodus simulator morphometric analysis. Numbers represent the landmark number designations used for the morphometric analysis: 1, ventral termination of the rostral adcarinal ridge; 2, dorsal termination of the carina; 3, ventral termination of the caudal adcarinal ridge; 4, rostral termination of the most ventral transverse ridge on the rostral side; 5, caudal termination of the most ventral transverse ridge on the rostral side; 6, rostral termination of the most ventral transverse ridge on the caudal side; 7, caudal termination of the most ventral transverse ridge on the caudal side; 8, rostral termination of the transverse ridge closest to the point of maximum curvature along the rostral platform margin on the rostral side. 9, caudal termination of the transverse ridge closest to the point of maximum curvature along the rostral platform margin on the rostral side; 10, rostral termination of the transverse ridge closest to the point of maximum curvature along the caudal platform margin on the caudal side; 11, caudal termination of the transverse ridge closest to the point of maximum curvature along the caudal platform margin on the caudal side; 12, rostral termination of the most dorsal transverse ridge on the rostral side; 13, caudal termination of the most dorsal transverse ridge on the rostral side; 14, rostral termination of the most dorsal transverse ridge on the caudal side; 15, caudal termination of the most dorsal transverse ridge on the caudal side; 16, dorsal tip of the element; 17, midpoint along the rostral adcarinal ridge; 18, midpoint along the caudal adcarinal ridge.

Morphotype membership for the Idiognathodus simulator group was determined on the basis of element shape, presence of an eccentric groove and lobe group membership. These criteria were chosen because these features are typically used for the discrimination of Idiognathodus species. Nearly all specimens could be confidently categorized as one of five morphotypes that have an eccentric groove. Idiognathodus luganicus (Kozitskaya 1978) has no lobes. Idiognathodus lateralis sp. nov. and I. praenuntius (Chernykh 2005) have one caudal lobe. Idiognathodus praenuntius is distinguished from I. lateralis sp. nov. by the higher degree of symmetry between the platform margins and a more medial position of the eccentric groove.

Idiognathodus auritus (Chernykh 2005) has both caudal and rostral lobes. Idiognathodus simulator (Ellison 1941) has a caudal adcarinal ridge that is separated from the caudal platform margin by the caudal adcarinal trough.

Step 3. Procrustes superimposition: A generalized Procrustes superimposition procedure translates, scales and rotates all specimens to their best mutual fit, and is used to remove the shape bias affiliated with specimen size, image position and rotation. The centroid size is defined as the square root of the summed Euclidean distances from each landmark to the specimen centroid. All specimen centroids are centered, and then all individuals are scaled to a common centroid size and rotated to a best fit. A “best fit” composite represents the shape created when using the mean values for each landmark (i.e., the landmark centroids) and essentially provides an “average” shape for the data set. Results from the Procrustes analysis were compared with results from a resistant-fit theta-rho analysis (RFTRA) to validate the assumption of landmark homogeneity in the Procrustes analysis. Getting similar results for both analyses informs the investigator that no one single landmark is overwhelmingly variable and effecting the location of other landmarks. Procrustes analysis is an important preliminary step because once the shape, scaling, and orientation noise is removed, the only variation that should remain between specimens is their true shape variation. In summary, this procedure provides an average shape reconstruction for the data set, and this information is required for the subsequent procedures and analyses. Application and theory of superimposition techniques are summarized by Zelditch et al. (2004), and are discussed in more mathematical detail in Rohlf and Slice (1990).

Step 4. Generating a covariance matrix: A covariance matrix can be generated after performing a Procrustes analysis. This is accomplished by calculating the sums of square roots and cross products (SSCP) of the deviations from the column means, which are the Procrustes coordinates (the landmark coordinates associated with the average shape; the landmark centroids), and the deviations would be the SSCP's of the deviations in the X and Y coordinates between an individual specimen and the Procrustes coordinates. These values can be used to compare individuals with the mean form calculated from the Procrustes analysis. The importance of this step is that for each specimen the raw coordinate values have been replaced with the deviations from the “mean” shape calculated from the Procrustes analysis, thus constructing a new data matrix that only contains values pertaining to variation in shape.

Step 5. Eigen analyses: Eigen analyses are statistical analyses that use vector geometry to determine the magnitude and direction of co-linear variation in multi-dimensional space. Eigenvectors and eigenvalues are calculated from either a covariance or a correlation matrix. The eigenvector is the vector used to determine the direction of maximum linear variation, and the corresponding eigenvalue is the amount of variance represented by the eigenvector. There will be as many eigenvectors present as there are statistical dimensions available, i.e., the rank of the matrix. The first eigenvector is the vector that best fits the linear variation between all the data in multidimensional space, and it has the largest eigenvalue. The second eigenvector is the vector that best fits the variation not accounted for by the first. Each successive eigenvector represents variation not accounted by the previous eigenvectors. This means that these eigenvectors are mutually orthogonal, have a hierarchy of significance, and when summed together describe 100% of the original data. No information is lost in these analyses and because of their statistical properties it is possible to deconstruct, dissect, or reconstruct the original matrix.

The two eigen analyses used for morphometric analysis in this study are principal component analysis (PCA) and canonical variate analysis (CVA). Principal component analysis (PCA) is an eigen analysis that calculates the linear variation within a data set under the assumption that all specimens belong to a single group. PCA provides composite variables that best represent the variation among all specimens in the original matrix. A canonical variate analysis (CVA) is an eigen analysis similar to PCA, but differing in that CVA is a discriminatory analysis that calculates the linear variation that best discriminates between multiple groups of variables, whereas PCA assumes all specimens belong to the same group and then describes the linear variation within that group. CVA requires group membership to be determined prior to analysis and assumes that group membership was correctly predetermined for the analysis. The number of eigenvectors for a CVA is dependent on the number of groups, and the independence, or mutual orthogonality, of those groups. For example, if a CVA is calculated for two independent groups there will only be one eigenvector representing the linear direction of the most discrimination between the groups in multi-dimensional space. There will always be one less eigenvector than there are number of independent groups. Further information and examples of eigen analyses being used for morphometric studies can be found in Douglas et al. (2001), Depecker et al. (2006), and Strauss (2010).

Step 6. Using eigenvectors to interpret morphological variation: Eigenvectors are calculated from the covariance matrix and are used in both the principal component analysis and canonical variate analysis. These vectors are typically called principal components (PC's) when used in a principal component analysis or canonical variates (CV's) when used in a canonical variate analysis. Each landmark coordinate for each specimen has a scaling for every eigenvector, and each individual specimen has a single score for each eigenvector. The eigenvector score for each specimen is calculated by taking the sum of the products from each raw landmark coordinate multiplied its respective eigenvector scaling. For example, the PC1 score for specimen A would be the sum of the products of each raw landmark coordinate multiplied by its respective PC1 scaling:

Because the eigenvectors relate to the original data it is also possible to model what a shape would look like by using scores of a chosen PC or CV. Any PC or CV values of zero will result in a landmark configuration that is identical to the average shape created by the Procrustes analysis. Changing the PC or CV value will result in the landmarks shifting from their original positions, and the direction and magnitude of that change is dependent on the PC or CV landmark coordinate scalings for that particular eigenvector. This allows the user to see the spatial change between landmarks as they change in their PC or CV values along a specific eigenvector. This can be used to determine what morphological changes occur along each eigenvector. For example, if negative values of PC1 resulted in a narrow platform element, than positive values of PC1 would result in a wider platform element and we can conclude that change along the PC1 vector corresponds to changes in platform width. This method is used to create wireframe models that illustrate what a theoretical form would look like at a particular location on any chosen eigenvector. These wireframes are included with the two-dimensional PC and CV cross plot figures discussed in the upcoming section and are used to help determine what morphological changes in the P₁ elements correspond with variation along a particular PC or CV eigenvector.

Results

Chirality shape variation.—Principal component analysis of specimens of the Idiognathodus simulator group resulted in 32 principal components (PC's) with the first two PC's representing 62.3% of the total variance (Fig. 5). Shape variation along PC1 corresponds with the ventral-dorsal shifting of the point of maximum curvature on the rostral platform margin, the most dorsal transverse ridges along both platform margins, and changes in length of both adcarinal ridges. Shape variation along PC2 corresponds with the inverse ventral-dorsal shifting of the points of maximum curvature on the platform margins, the caudal-rostral shifting of the dorsal tip of the element, and variation in adcarinal ridge length.

When the specimens are coded based on chirality, it is apparent that the sinistral and dextral elements occupy separate regions of morphospace (Fig. 5). To investigate this segregation, a canonical variate analysis was executed using chirality as the classification criterion. This analysis showed a very strong separation between sinistral and dextral elements with nearly no overlapping CV values between sinistral and dextral elements (Fig. 6). The canonical variate analysis shows that the dextral elements have shorter adcarinal ridges, and a more ventral location of maximum curvature along the rostral platform margin, resulting in a more triangular-shaped platform for dextral elements, and a more elongate diamond-shaped platform for sinistral elements. The relative size variation between the rostral platform region and the caudal platform region also tends to be much greater in dextral elements. The segregation between sinistral and dextral elements on the principal component cross-plot indicates that the shape variation associated with chirality membership is the greatest shape variation in the data set. This shows that the shape difference between sinistral and dextral elements will overwhelm any variation associated with other characters in a principal component analysis. Because of this chirality bias, sinistral and dextral elements are analyzed separately in the following analyses of shapes associated with lobe group membership.

Fig. 5.

Principal component (PC) cross-plot and wireframes for all grooved specimens. A. Principal component cross plot of the first two principal components, labeled with their corresponding eigenvalues (blue, sinistral; red dextral). B. Black wireframes, the resulting form with values of the corresponding principal component values; gray outline, the mean form provided by the preliminary Procrustes fit.

Fig. 6.

Canonical variation plot and wireframes for all grooved specimens, using chirality membership as the group-defining criterion. Gray wireframe, the mean form provided by the preliminary Procrustes fit, and blue/ red wireframes, form achieved with values of the corresponding canonical variate; blue, sinistral; red dextral.

Lobe group shape variation.—In order to investigate the shape variation associated with lobe group membership, shape variation associated with chirality asymmetry was removed by performing separate canonical variance analyses for sinistral and dextral elements. Crossplots of the three canonical variates showed a gradational separation of lobe groups in two-dimensional morphospace for sinistral elements (Fig. 7A). The lobeless (Idiognathodus luganicus) and disconnected ridge (I. simulator) morphotypes are primarily constrained to negative values of CV1, whereas the two-lobed morphotype (I. auritus) is confined to positive values of CV1. The one-lobed morphotypes (I. praenuntius and I. lateralis sp. nov.) fill the morphospace between them, falling between CV1 values of -2 and 2. The lobeless (I. luganicus) and disconnected ridge morphotypes (I. simulator) are moderately separated along CV2 and are separated further along CV3. Variation along CV1 corresponds to constriction and expansion of the adcarinal ridge midpoints. Variation along CV2 corresponds with caudal shifting of both adcarinal ridges and some ventral-dorsal shifting of the point of maximum curvature on the rostral platform margin. Variation along CV3 corresponds with change in the orientation of the adcarinal ridges. Positive values of CV3 correspond with a caudally oblique orientation for both adcarinal ridges and negative values of CV3 correspond with adcarinal ridges that are sub-parallel to the platform margins.

Canonical variate analysis of the lobe groups within dextral elements also shows segregation of groups in morphological space (Fig. 7B). Note that the two-lobe character state (Idiognathodus auritus) is very rare in dextral elements and has meager representation in the analysis. The crossplot of the first two canonical variates (82.2% of the total variance) shows a gradational segregation between the one-lobe morphotypes (I. praenuntius and I. lateralis) and all other groups along the CV1 axis. A separation exists between the lobeless (I. luganicus) and disconnected-ridge (I. simulator) morphotypes along the CV2 axis. Positive values of CV1 show a strong concavity along the caudal edge of the element that spans from the curvature maxima of the caudal platform margin to the ventral tip of the adcarinal ridge. This constriction is also present on the sinistral elements. However, in the sinistral elements the constriction did not extend as far along the platform margin. Variation along CV1 corresponds with concavity of the caudal edge of the element, caudal-rostral shifting of the dorsal tip, as well as obliquity changes and the dorsal-ventral shifting of the rostral transverse ridges. CV2 values also vary with the concavity of the caudal edge and the flaring of the rostral adcarinal ridge. The cross plot of CV1 and CV3 (∼68% of the total variance; Fig. 7B) shows weak separation between groups suggesting that most of the separation between specimens in morphospace occurs along CV1 and CV2.

The adcarinal ridges appear to constrict with the presence of lobe growth on all specimens. Dextral elements experience much more dorsally-ventrally extensive platform margin constriction and adcarinal ridge concavity in the presence of lobe growth. Specimens with disconnected adcarinal ridges (Idiognathodus simulator), for both sinistral and dextral elements, show that the midpoint of the adcarinal ridge flares outward from the carina.

Morphotype shape variation.—Idiognathodus luganicus has an eccentric groove and no lobe development. A crossplot of the first two principal components reveals some separation in morphospace between sinistral and dextral elements, with dextral elements falling in the lower left half of the graph, and sinistral elements in the upper right portion (Fig. 8A). A canonical variate analysis supports this shape distinction between sinistral and dextral elements (Fig. 8B). Sinistral elements have relatively straight platform margins, with the locations of maximum curvature in a medial position along both margins. However, the rostral margin extends farther laterally than the caudal margin. The rostral adcarinal ridge is slanted toward the carina, and the caudal adcarinal ridge tends to extend sub-parallel to the carina and the caudal platform margin, giving the element a relatively straight caudal edge and a broad, gradually curved rostral edge. The dorsal termination of the carina is typically ventral of the most ventral transverse ridges. Dextral elements also exhibit a relatively straight caudal element edge, sub-parallel to the carina, but differ from the sinistral elements primarily owing to shape variation on the rostral platform margins. The point of maximum curvature along the rostral platform margin is typically located in a more ventral position on the dextral elements, usually near the most ventral ridge. The most ventral ridge on the rostral side of dextral elements is located in a more ventral position than in the sinistral elements, and frequently lies beyond the dorsal termination of the carina. The ventral extension of the rostral platform region and the point of maximum curvature result in a wide ventral region that tapers dorsally. This wide ventral region is also associated with a steep deflection of the rostral adcarinal ridge toward the carina.

Fig. 7.

Canonical variation (CV) cross-plots and wireframes for all grooved sinistral (A) and dextral (B) elements, using lobe membership (red, 0 lobes; blue, 1 lobe; green, 2 lobes; purple, dicconnected ridge) as the group-defining criterion. Gray wireframes, mean form provided by the preliminary Procrustes fit; black wireframes, form achieved with values of the corresponding canonical variate.

Idiognathodus lateralis sp. nov. has an eccentric groove and a caudal lobe. A crossplot of the first two principal components and a canonical variate analysis of chirality reveal that this morphotype contains asymmetrical dextral and sinistral elements (Fig. 9A, C). Both sinistral and dextral elements have slightly concave caudal adcarinal ridges and ventral platform margins. The maximum curvature of the rostral platform margin is shifted farther ventrally in the dextral elements than the sinistral elements and gives the rostral margin a sharper, more triangular edge. This change in the rostral platform region is the source of the most variation between the sinistral and dextral elements and comprises much of the variation along the PC2 axis. Both sinistral and dextral elements vary in the location of maximum curvature along their caudal platform margins. Both adcarinal ridges flare caudally for sinistral and dextral elements, but the group shows variation in the angle of deflection. The rostral platform region is larger than the caudal platform region. However, in some dextral elements the most ventral ridge on the rostral side is located ventral to the dorsal termination of the carina, similar to the dextral elements of I. luganicus.

Fig. 8.

Cross-plot of the first two principal components (labeled with their corresponding eigenvalues) and wireframes (A) and canonical variation plot (using chirality membership as the group defining criterion) and wireframes (B) for Idiognathodus luganicus. Black wireframes, form with values of the corresponding principal component values; gray wireframes, mean form provided by the preliminary Procrustes fit; blue/red wireframes, form achieved with values of the corresponding canonical variate; blue, sinistral; red, dextral.

Fig. 9.

Principal component cross-plots of the first two principal components (labeled with their corresponding eigenvalues) and wireframes (A, B) and canonical variation plots (using chirality membership as the group defining criterion) and wireframes (C, D) for Idiognathodus lateralis (A, C) and Idiognathodus praenuntius (B, D). Black wireframes, form with values of the corresponding principal component values; gray wireframes, mean form provided by the preliminary Procrustes fit; blue/red wireframes, form achieved with values of the corresponding canonical variate; blue, sinistral; red, dextral.

Idiognathodus praenuntius has an eccentric groove and a caudal lobe, but is distinguished from I. lateralis sp. nov. by having a more medial location of the eccentric groove, and more symmetrical platform margins. This morphotype exhibits asymmetrical pairs, as shown by the results of the principal component analysis and a canonical variate analysis (Fig. 9B, D). A broader rostral platform margin with a ventrally shifted point of maximum curvature is characteristic of the dextral elements and is the major difference between the sinistral and dextral elements. The relative length of the platform is typically shorter for both sinistral and dextral elements of this morphotype and the adcarinal ridges tend to maintain a sub-parallel orientation to the carina, although some elements, primarily dextral elements, may flare caudally. The caudal platform margin tends to have a much more dorsal location of maximum curvature than other morphotypes, giving these specimens a more symmetrical and elliptical platform shape.

Specimens having an adcarinal ridge disconnected from the platform margin by the adcarinal trough are Idiognathodus simulator. In some larger specimens of I. simulator, a small lobe is developed on the rostral margin, but these specimens were not placed in I. auritus, owing to the disconnected adcarinal ridge on the caudal side. Principal component analysis suggests shape variation between the sinistral and dextral elements (Fig. 10A), and a canonical variate analysis confirmed this (Fig. 10C). The trend of a more ventrally shifted point of curvature maxima along the rostral platform margin is more common in the dextral elements and is the primary shape difference between the sinistral and dextral elements for this morphotype. Another asymmetrical feature of this morphotype is the orientation of the disconnected adcarinal ridge. For sinistral elements this ridge tends to be oriented at a more oblique angle to the carina than in the dextral elements. In most sinistral elements, the ventral part of the caudal platform margin flares rostrally at an angle sub-parallel to the orientation of the disconnected adcarinal ridge. Many dextral elements have a caudal platform margin and a disconnected ridge that maintain a sub-parallel orientation to one another and to the carina. For dextral elements, the curvature maximum is located near the most ventral ridge on the caudal side, and near the platform margin midpoint on the rostral side.

Specimens with an eccentric groove and lobe development on both the rostral and caudal sides are Idiognathodus auritus. Dextral forms are rare, but the few dextral elements in this analysis appear to segregate distinctly from the sinistral elements in a principal component analysis (Fig. 10B) as well as in a canonical variate analysis (Fig. 10D). The most distinguishing character between chiralities appears to be the position of maximum curvature along the rostral platform margin, as well as a shortening of the adcarinal ridges in dextral forms. All specimens tend to exhibit concavity of the ventral portion of the platform margins and the adcarinal ridges, forming a rostrum. Although the dextral elements have the maximum curvature of the rostral platform margin shifted ventrally, most specimens have their maximum curvature at or near mid-length, giving these specimens a platform shape similar to I. praenuntius. On the basis of platform shape and the position of the eccentric groove, some of these specimens are similar to those included in I. praenuntius, but differ in the presence of a rostral lobe.

Fig. 10.

Principal component cross-plots of the first two principal components (labeled with their corresponding eigenvalues) and wireframes (A, B) and canonical variation plots (using chirality membership as the group defining criterion) and wireframes (C, D) for Idiognathodus simulator (A, C) and Idiognathodus auritus (B, D). Black wireframes, form with values of the corresponding principal component values; gray wireframes, mean form provided by the preliminary Procrustes fit; blue/red wireframes, form achieved with values of the corresponding canonical variate; blue, sinistral; red, dextral.

In all the shape analyses discussed in previous sections, the size aspect was removed in the preliminary Procrustes analysis. To study allometry, the centroid size calculated from the raw landmark data during the Procrustes analysis was recovered from the data matrix and used as a measure of size. The centroid for each specimen is the center point that minimizes most the total summed distance from each landmark to the centroid. The centroid size is the sum of the square roots of the distances between each landmark to the image centroid. Using this as a measure of size is different than a measurement of length or width, as the lateral expansion of any landmark will increase the centroid size. However, comparison with the photographed specimens indicates that a centroid size of 800–1000 pixel units corresponds to a specimen with a platform length of approximately 0.3–0.4 mm.

A plot for the centroid size of each specimen on the y-axis, and their morphotype membership along the x-axis was created for sinistral and dextral specimens of the Idiognathodus simulator group (Fig. 11). At a small size, approximately less than 0.3 mm in platform length, all sinistral and dextral elements of the I. simulator group lack lobes, and appear similar to one another. For the sinistral elements, lobes can be distinguished at slightly larger sizes (greater than 0.3 mm platform length), but the presence of two lobes is less apparent at smaller sizes (platform lengths between 0.3 and 0.4 mm). In dextral elements lobes may not develop until a larger size than in sinistral elements. The morphotype I. simulator appears to show strong allometric bias in both sinistral and dextral elements, for it cannot be recognized in the smaller specimens.

Fig. 11.

Cross-plot of morphotype membership for each grooved sinistral (A) and dextral (B) specimen on the x-axis and their centroid size along the y-axis. Centroid size is not a measure of length, because the outwards movement of any landmark from the centroid will cause it to increase, but for the sake of providing reference, specimens with a platform length less than 0.4 mm generally had a centroid size below 900.

Morphologic variation by locality.—Species of the Idiognathodus simulator group occur through the Heebner Shale in all three sections. The P₁ elements are most abundant in the black fissile shale. In black shale samples from the lower portion of the Heebner Shale, the I. simulator group makes up 80 to 90 percent of the conodont elements present, with the remaining portion being composed of Idioprioniodus and/or Streptognathodus elements (SOM 1: tables 2, 4, and 6). Idiognathodus luganicus and I. lateralis sp. nov. are the most common morphotypes and I. simulator is relatively uncommon. The only member of the I. simulator group that shows any stratigraphic or geographic preference is I. praenuntius, which appears more commonly in the lower half of the Heebner Shale in all three sections and is nearly absent northward at the I-229 roadcut. Canonical variate analysis by locality was run for both sinistral and dextral P₁ elements of the Idiognathodus simulator group. In both analyses locality membership appears to separate into partially distinctive, but moderately overlapping, regions of two-dimensional morphospace (Fig. 12). Sinistral elements vary in adcarinal ridge length and curvature along CV1, as well as in lengthening and shortening of the platform, and thinning or thickening of the platform region caudal of the eccentric groove. Sinistral element shape along the CV2 axis corresponds with adcarinal ridge length as well as a dorsal-ventral shifting of the curvature maxima along both platform margins. CV1 values for dextral elements vary primarily with overall platform narrowing and widening, length changes of the platform and the adcarinal ridges, as well as the curvature of the caudal platform margin, and the direction of caudal adcarinal ridge deflection. Increasing positive values along the CV2 axis indicate longer adcarinal ridges and a dorsal shifting of the curvature maxima on both platform margins, with the opposite holding true for negative CV2 values. The two morphologic features most associated with change in locality are adcarinal ridge and relative platform size variation in both the sinistral and dextral elements. For both elements, the subpopulation from the Sedan locality tends to have longer adcarinal ridges, and shorter, wider platform regions. The I-229 specimens, as a whole, exhibit longer platforms and more oblique transverse ridges.

Fig. 12.

Canonical variation (CV) cross-plots and wireframes for all grooved sinistral (A) and dextral (B) elements, using locality (red, Clinton; green, I-229; blue, Sedan) as the group-defining criterion. Gray wireframes, mean form provided by the preliminary Procrustes fit; black wireframes, form achieved with values of the corresponding canonical variate.

Discussion

The geometric morphometric analyses of the five morphotypes of the Idiognathodus simulator group demonstrate that the sinistral and dextral P₁ elements occur as asymmetrical pairs in each morphotype, despite differences in lobes and platform shape. The dextral elements are typically widest near the ventral end of the platform, giving these elements a more triangular shape than their curved sinistral counterparts. The greater width of the dextral element is due to the lateral expansion of the rostral platform margin during growth. The difference in width between the sinistral and dextral elements increases with size. The degree of asymmetry also increases as the elements increase in size and the dextral elements transform greater from the juvenile morphology than the sinistral elements. Dextral elements show a greater allometric bias for lobe development and adcarinal ridge isolation, which suggests that sinistral elements may prove more useful in diagnosing species membership in smaller specimens.

P₁ element asymmetry is not unknown in Idiognathodus and Streptognathodus species and has been expressed by different species groups at different times during the Pennsylvanian. Idiognathodus species with strong P₁ element asymmetry are known from Bashkirian strata (e.g., Idiognathodus sinuosus Ellison and Graves, 1941; see Lane and Straka 1974). However, most Moscovian and Kasimovian species appear to have had more symmetrical, mirror-image P₁ elements. The I. simulator group represents the redevelopment of P₁ element asymmetry in one lineage during the late Kasimovian and early Gzhelian (see Systematic palaeontology section), before the extinction of the genus in the Gzhelian. Then, near the end of the Gzhelian and into the Asselian (Early Permian) species of Streptognathodus independently developed asymmetrical P₁ element pairs (Boardman et al. 2006).

The five morphotypes distinguished in this study are considered to represent different species that can be distinguished on the presence of caudal and rostral lobes, character of the adcarinal ridges, and platform shape. These are the same features that have been used to distinguish co-occurring species in Idiognathodus and Streptognathodus (e.g., Boardman et al. 2006; Chernykh 2005, 2012; Rosscoe and Barrick 2009, 2013). The most common species in the Heebner Shale are I. lateralis sp. nov. and I. luganicus, whereas I. simulator, I. auritus, and I. praenuntius are less common. All of these species occur throughout the Heebner Shale at all three localities. Idiognathodus simulator is not the most abundant species of the I. simulator group, but does consistently occur where the I. simulator group is present.

Previously, the name Idiognathodus simulator was used to designate a large variety of P₁ elements that share a caudal lobe and an eccentric groove (Barrick et al. 2008; see Systematic palaeontology section). Based on features of the holotype and the geometric morphological analysis, the concept of I. simulator is restricted to only P₁ elements with a high, elongate and isolated caudal adcarinal ridge. The adcarinal ridge extends beyond the end of the platform margin into the region where a caudal lobe would be present. The adcarinal ridges and platform margins form the outer platform boundaries of the P₁ element and because a lobe is defined as growth outside the boundaries, this feature cannot be classified as a lobe. The new species I. lateralis sp. nov. is erected here to designate specimens that have a caudal lobe and a continuous connection between the caudal platform margin and the caudal adcarinal ridge. Discrimination between these two taxa is justified by the morphological differences through most ontogenetic stages of growth. However, at platform sizes smaller than 0.4 or 0.3 mm in maximum length, species of the I. simulator group become difficult to distinguish. This is because I. simulator has a juvenile state that shows weak separation between the adcarinal ridge and the platform margin. At these sizes it may be difficult to distinguish whether or not the first caudal node is located on a lobe or is protruding out as an extension of the adcarinal ridge. When the platform gets larger than 0.3 or 0.4 mm in length this distinction can be more confidently made. The more precise definition of I. simulator should aid in the accurate correlation of the base of the Gzhelian, once a Global Boundary Stratotype Section and Point (GSSP) is chosen (Heckel et al. 2008). This may help avoid the confusion that has arisen recently at other chronostratigraphic boundaries where conodont species were not sufficiently described before the GSSPs were chosen (e.g., Siphonodella sulcata for the base of the Carboniferous [Kaiser and Corradini 2011; Corradini et al. 2013]; Declinognathodus noduliferous for the base of the Pennsylvanian [Sanz-López et al. 2006; Sanz-López and Blanco-Ferrera 2013]).

Because the Oread cyclothem, which includes the Heebner Shale, represents only a brief interval of time, 100 000 to 400 000 years, related to orbital eccentricity forcing (e.g., Heckel 1986, 2008; Davydov et al. 2010), we cannot resolve the origin of Idiognathodus simulator and related species, nor the order in in time in which these species appeared. The Oread cyclothem is a major cyclotherm, one of few Midcontinent cyclothems in the latest Kasimovian and early Gzhelian in which a far offshore, organic-rich black shale developed (Heckel 1999). The underlying cyclothems in the Midcontinent region are minor to intermediate cyclothems that have produced Streptognathodus-dominated faunas and rare examples of Idiognathodus (Barrick et al. 2013a). It is possible that I. simulator may have evolved earlier in time than the Oread cyclothem and that its first occurrence in the Midcontinent region is the result of the major flooding event that produced the cyclothem rather than the speciation event that gave rise to I. simulator. The stratigraphic record in other regions (Moscow Basin; Alekseev et al. 2009; southern Urals, Chernykh 2005, 2012) also shows that the I. simulator group appears abruptly in these sections, with little evidence for the line of descent from ancestral forms.

The morphometric results strongly support the hypothesis that the growth of lobes correlates with concavity along the platform margins. This concavity occurs along the entire dorsal-ventral extent of the lobe. The maximum concavity occurs near the same point where the lobe is widest. For example, the lobeless species Idiognathodus luganicus has the straightest platform margins and adcarinal ridges of all the Idiognathodus simulator group species from the Heebner Shale. On the other extreme, the species that exhibit lobe growth on both rostral and caudal sides, such as I. auritus, have the most concave and curved platform margins and adcarinal ridges. Additionally, species that only have lobe growth on the caudal side, I. lateralis sp. nov., I. praenuntius, and I. simulator only have platform concavity on the caudal side. The rostral side of these single-lobed species appears relatively straight or convex and is similar to the rostral platform margins of lobeless species. The correlation of increased concavity along the platform margins of specimens with lobes is supported by the results of the lobe-group morphometric analysis for both sinistral and dextral elements of the I. simulator group (Fig. 7). All specimens of Idiognathodus studied here lack lobes at a small size (platform lengths less than 0.3 mm) and the lobes appear at a larger ontogenetic stage. The platform margin of smaller specimens may be slightly concave or the adcarinal ridge may flare outwards prior to the development of nodes. During this stage a “phantom lobe” is present, which is a growth outside of the platform margin that is of nearly equivalent oral height as the main platform surface, but is not yet ornamented with a single node.

Because lobe size, shape and nodosity are so diverse in many species of Idiognathodus and Streptognathodus and typically correspond with changes in platform shape, the development of particular lobe and platform architectures may be the result of specialization in the processing of particular prey items (Purnell et al. 2000; Rosscoe and Barrick 2009). Because the lobes are of equivalent oral height as the platform surface, and because the platform surface is interpreted to be the surface in contact with prey items (Purnell and Jones 2012; Jones et al. 2012) they must play a role in the processing of prey items. If this hypothesis is correct, than the continued use of lobe and platform shape characters in taxonomic discrimination is still a valid practice, as it seems reasonable to believe that difference species would have fed on a different selection of prey.

The functional significance of asymmetry in P₁ element pairs has been shown to allow interlocking during element occlusion in carminiscaphate elements of Idiognathodus (Donoghue and Purnell 1999) and Gnathodus (Martínez-Pérez et al. 2014b). However, Martínez-Pérez et al. (2014a) show that asymmetrical pairs of carminate elements of Pseudofurnishius murcianus rigidly interlock and leave little room for element rotation, and do not occlude in the same way as do Idiognathodus and Gnathodus species. This suggests that different groups of conodonts had unique functional relationships between elements of the apparatus related to different modes of food processing, Accurate quantitative analysis of the shape differences between sinistral and dextral P₁ elements, such as that used here, will be essential for future studies of occlusion or element “matching”.

Conclusions

Landmark-based geometric morphometric techniques, combined with multivariate eigen analyses, have allowed for the two dimensional spatial analysis of landmarks on P₁ elements of the conodont genus Idiognathodus. These data can be visually represented as wireframes and used to compare quantitative statistical variation to qualitative P₁ element shape variation. Principal component analysis was used to determine variation within a particular group of data and canonical variate analysis was used to determine variation between particular groups of data. The morphometric procedure developed here for the analysis of Idiognathodus P₁ element shape variation can be easily modified and used in the analysis of P₁ elements of other conodont taxa.

All species of the Idiognathodus simulator group (I. simulator, I. auritus, I. lateralis sp. nov., I. luganicus, and I. praenuntius) have asymmetrical P₁ element pairs. Sinistral elements are narrow, curved and elongate. Dextral elements are straight, triangular, and have a wider rostral platform area. Species of the I. simulator group are defined by platform shape and the presence of lobes. Some I. simulator group species share juvenile P₁ element morphologies and do not reach a diagnostic growth stage until the platform is longer than approximately 0.4 mm. Dextral elements of the I. simulator group show more allometric bias in the development of lobes and the isolation of the caudal adcarinal ridge. This suggests that sinistral elements may be better for species discrimination among smaller specimens. The concept of I. simulator has been restricted to asymmetrically paired P₁ elements with an eccentric groove and an isolated caudal adcarinal ridge, which should ensure consistent correlation of the Kasimovian Gzhelian boundary.

Systematic palaeontology

Illustrated specimens are reposited at the University of Iowa Paleontology Repository (SUI). All specimens included in the morphometric analyses were assigned a separate morphometric identification number (TT). Each locality has two abundance tables, one that gives the Idiognathodus elements as morphotype groups separated by chirality (SOM 1: tables 1, 3, and 5), and another that lists Idiognathodus species (SOM 1: tables 2, 4, and 6).

Fig. 13.

Conodont P₁ elements of Idiognathodus simulator (Ellison, 1941) from the Heebner Shale, Gzhelian (Late Pennsylvanian). A. SUI 141025 (TT0312); Sedan 19. B. SUI141026 (TT0248); Clinton 14. C. SUI 141027 (TT0244); Clinton 12. D. SUI 141028 (TT0252); Sedan 11. E. SUI 141029 (TT0149); Sedan 20. F. SUI 141030 (TT0103); Sedan 15. G. SUI 141031 (TT0313); Sedan 16. H. SUI 141032 (TT0192); Sedan 16. I. SUI 141033 (TT0247); Clinton 13. J. SUI 141034 (TT0077); I229-82. K. SUI 141035 (TT0292); Clinton 12. L. SUI 141036 (TT0157); Clinton 20. M. SUI 141037 (TT0291); Clinton 12. N. SUI 141038 (TT0193); Sedan 16. O. SUI 141039 (TT0076); I229-82. P. SUI 141040 (TT0290); Clinton 12. Q. SUI 141041 (TT0158); I229-92.

Fig. 14.

Conodont P₁ elements of Idiognathodus auritus (Chernykh, 2005) from the Heebner Shale, Gzhelian (Late Pennsylvanian). A. SUI 141042 (TT0314); Sedan 17. B. SUI 141043 (TT0311); Sedan 24. C. SUI 141044 (TT0251); Sedan 11. D. SUI 141045 (TT0222); Clinton 12. E. SUI 141046 (TT0251); Sedan 11. F. SUI 141047 (TT0235); Sedan 11. G. SUI 141048 (TT0256); Sedan 11. H. SUI 141049 (TT0095); I229-83. I. SUI 141050 (TT0093); Clinton 21. J. SUI 141051 (TT0094); I229-83. K. SUI 141052 (TT0221); Clinton 12. L. SUI 141053 (TT0169); Sedan 23. M. SUI 141054 (TT0167); Sedan 23.

Fig. 15.

Conodont P₁ elements of Idiognathodus lateralis sp. nov. (A–M) and Idiognathodus praenuntius (Chernykh, 2005) (N–T) from the Heebner Shale, Gzhelian (Late Pennsylvanian). A. SUI 141062 (TT0228); Sedan 11. B. SUI 141063 (TT0233); Sedan 11. C. SUI 141064 (TT0050); Clinton 20. D., SUI 141065 (TT0019); Clinton 22. E. SUI 141066 (TT0168); Sedan 23. F. SUI 141067 (TT0261); Sedan 11. G. SUI 141068 (TT0260); Sedan 11. H. SUI 141069 (TT0064); Clinton 29. I. Holotype, SUI 141070 (TT0039); Clinton 19. J. SUI 141071 (TT0049); Clinton 28. K. SUI 141072 (TT0034); Sedan 20. L. SUI 141073 (TT0185); Clinton 37. M. SUI 141074 (TT0170); Sedan 20. N. SUI 141075 (TT0214); Clinton 18. O. SUI 141076 (TT0066); Clinton 27. P. SUI 141077 (TT0055); Sedan 23. Q. SUI 141078 (TT0176); Clinton 19. R. SUI 141079 (TT0188); Clinton 29. S. SUI 141080 (TT0269); Sedan 11. T. SUI 141081 (TT0264); Sedan 11.

Fig. 16.

Conodont P₁ elements of Idiognathodus luganicus (Kozitskaya, 1978) from the Heebner Shale (Gzhelian, Late Pennsylvanian). A. SUI 141094 (TT0200); Sedan 11. B. SUI 141095 (TT0209); Clinton 12. C. SUI 141098 (TT0285); Clinton 12. D. SUI 141096 (TT0284); Clinton 12. E. SUI 141097 (TT0141); Clinton 28. F. SUI 141099 (TT0018); Clinton 15. G. SUI 141100 (TT0026); I229-83. H. SUI14101 (TT0016); Clinton 14. I. SUI 14102 (TT0119); Sedan 20. J. SUI 141103 (TT0118); Sedan 20. K. SUI 141104 (TT0102); Sedan 12.