Introduction

At the end of the Triassic, ichthyosaurs underwent a major diversity (Motani 2005) and disparity (Thorne et al. 2011) drop. From the latest Triassic (late Rhaetian) onward, lineages of the first ichthyosaurian radiation were replaced by the derived, “fishlike” parvipelvian ichthyosaurs, with the rapid successive appearances of several lineages during the latest Triassic (Fischer et al. 2013). Although Triassic ichthyosaurs filled various niches, as highlighted by tooth and mandibular morphology, with piercing, crushing (Massare 1987, 1997; Massare and Callaway 1990), giant gulping forms (the shastasaurids, Nicholls and Manabe 2004; Sander et al. 2011; but see Motani et al. 2013; Ji et al. 2013), and large macropredators (Motani et al. 1999; Fröbisch et al. 2006, 2013), the subsequent parvipelvian radiation comprised forms with less diverse diets (Sander 2000). In contrast to the first ichthyosaurian radiation, parvipelvians are not recognized to have included giant taxa (i.e., total body length over 15 m), the largest forms being represented by the genus Temnodontosaurus (e.g., McGowan 1996; Martin et al. 2012). The precise timing of the Late Triassic ichthyosaurian turnover (e.g., Thorne et al. 2011; Benson et al. 2012) is, however, unknown because of inadequate sampling in upper Norian—Rhaetian strata.

The presence of a giant ichthyosaurian taxon in the Lower Jurassic of the UK has been suspected for some time, on the basis of large postcranial fragments (McGowan 1996), but such a giant form occurring after the demise of Triassic non-parvipelvians may challenge our understanding of their evolutionary history.

Here we describe a radius of exceptional size, collected at Penarth on the coast of south Wales near Cardiff, UK. This specimen is comparable in morphology and size to the radius of shastasaurids, and it is likely that it comes from a stratigraphic horizon considerably younger than the last definite occurrence of this family, the middle Norian (Motani 2005), although remains attributable to shastasaurid-like forms from the Rhaetian of France were mentioned by Bardet et al. (1999) and very recently by Fischer et al. (2014).

Institutional abbreviations.—BRLSI, Bath Royal Literary and Scientific Institution, Bath, UK; NHM, Natural History Museum, London, UK; NMW, National Museum of Wales, Cardiff, UK; SMNS, Staatliches Museum für Naturkunde, Stuttgart, Germany.

Other abbreviations.—TBL, total body length.

Geological setting

Stratigraphic horizon of the specimen .—The Penarth-Laver nock Point coastal cliff section where the specimen was found as a loose block exposes strata from the upper beds of the Mercia Mudstone Group Blue Anchor Formation (?late Norian—Rhaetian), through Penarth Group Westbury and Lilstock formations (Rhaetian), to the lower beds of the Lias Group Blue Lias Formation (early Hettangian) at the top of the cliff (see Waters and Lawrence 1987; Lawrence and Waters 1989; Warrington and Ivimey-Cook 1995; Benton et al. 2002; Suan et al. 2012 for detailed descriptions).

The specimen NMW95.61G.1 preserves a patch of dark grey-bluish, non-fissile muddy limestone matrix. This excludes as a source the red mudstone and light grey dolomitic beds of the Blue Anchor Formation, the dark shales and sandy bonebeds of the Westbury Formation, and the pale limestone, sandstone and shale beds of the Cotham and Langport members of the Lilstock Formation. However, it could indicate that the specimen came from either the dark grey-bluish limestone beds of the Rhaetian Westbury Formation or the Hettangian Blue Lias Formation (Hodges 1994).

The matrix contains several invertebrate macrofossils, in varying states of preservation that constrain its likely age. Poorly preserved internal moulds of small (<5 mm) gastropods are attributed, based on whorl morphology, to Allocosmia sp. which ranges from Middle Triassic to Early Jurassic; a similar age range is indicated by a specimen of Bakevellia sp., identified based on hinge structure, and a poorly preserved ?pteriid bivalve. However, a possible immature ostreid is indicative of the lower part of the Blue Lias Formation “pre-planorbis Beds” and Palaeonucula navis (Piette, 1858) (Fig. 1), which is not known from beds older than the Psiloceras planorbis Biozone (PH personal observation; Mander and Twitchett 2008).

Fig. 1.

A. Lithostratigraphy, biostratigraphy, and carbon-isotope stratigraphy of key Triassic-Jurassic sections illustrating the likely age of the specimen NMW95.61G.1 from Penarth, south Wales, UK. Organic and inorganic carbon isotope data of Penarth-Lavernock Point from Suan et al. (2012) and Korte et al. (2009); organic carbon isotope data of St. Audrie's Bay and Kuhjoch from Hesselbo et al. (2002) and Ruhl et al. (2009). The specimen was deposited several kyr after the end-Triassic mass extinction (initial neg. CIE = initial negative carbon-isotope excursion). Abbreviations: CM, Cotham Member; LM, Langport Member. B. Close-up photograph of the bivalve Palaeonucula navis (Piette, 1858) preserved in the embedding matrix of specimen NMW95.61G.1.

The Blue Lias Formation is exposed in the cliff section where the specimen was found. The lowermost beds of the Blue Lias Formation “pre-planorbis Beds” are immediately adjacent on the clifftop, but we would be inclined to dismiss these as they locally represent a very shallow subtidal to intertidal facies dominated by the bivalves Liostrea hisingeri (Nilsson, 1832) and Modiolus minimus (Sowerby, 1818). Further, the shallow waters of the pre-planorbis Beds could accommodate only strand-line type deposition of floating material, an unlikely scenario given the size and weight of NMW95.61G.1. The younger Hettangian Psiloceras planorbis Biozone strata crops out a short distance to the south, and cobbles and boulders of this are frequently carried northwards along the beach. As this is an offshore, deeper water facies, it is a stronger argument to suggest this as a source for specimen NMW95.61G.1. The Blue Lias Formation at Penarth has previously yielded well-preserved specimens of Leptonectes and Ichthyosaurus, now in the NMW.

Age and position relative to the end-Triassic extinction.—The Blue Lias Formation, the probable source of the specimen NMW95.61G.1, includes the Psiloceras planorbis Biozone and passes through to the Agassiceras scipionianum Sub-biozone of the Arnioceras semicostatum Biozone, making it Hettangian to Early Sinemurian in age. Nevertheless, the base of the Blue Lias Formation also includes the “pre-planorbis Beds”, the age of which is poorly biostratigraphically constrained. Indeed, the Triassic-Jurassic boundary has been defined at Kuhjoch in the Eiberg Basin in Austria in 2012 (Hillebrandt and Krystyn 2009) as the first occurrence of the ammonite Psiloceras spelae tirolicum Hillebrandt and Krystyn, 2009, a species that does not occur in the UK. As such, the position of the base of the Jurassic is not readily identifiable in the field within the Blue Lias Formation on the Penarth-Lavernock Point cliff section. Carbon isotope correlations suggest that the base of the Jurassic lies at or just below the base of a finely laminated shale unit (the “Paper Shales”) (Korte et al. 2009) indicating that the Blue Lias limestone beds (i.e., the likely source rock of the specimen NMW95.61G.1) were deposited several hundred thousand years (Ruhl et al. 2010) after the main end-Triassic extinction event (Fig. 1). Accordingly, the specimen postdates the main mass extinction event and associated carbon isotope excursion recorded at this locality (Mander et al. 2008; Suan et al. 2012). Most importantly, assignment to Triassic strata substantially older than the Rhaetian is precluded by their non-marine origin in the UK (Hounslow et al. 2004).

Fig. 2.

A–E. Specimen NMW95.61G.1, radius of possible shastasaurid ichtyosaur from the Lower Jurassic of Penarth, South Wales, in proximal (A), posterior (B), ventral or dorsal (C, D), and distal (E) views. F. Regression graph shows the relation between preflexural length versus radius height (both in cm) in selected ichthyosaurian specimens (see Supplementary Online Material available at  http://app.pan.pl/SOM/app60-Martin_etal_SOM.pdf). The stippled line corresponds to NMW95.61G.1. The outline is based on Kosch (1990) reconstruction of Shonisaurus popularis.

Systematic palaeontology

Table 1.

Dimensions (in cm) of the shastasaurid radius (NMW95.61G.1) from the Lower Jurassic of Penarth, Wales.

Discussion

Body size.—We estimate a minimum preflexural length of 12–15 m for the Welsh animal (Fig. 2), therefore implying a slightly larger total body length (TBL). This estimate is based on the size of the radius. We compiled body measurements (TBL versus radius height and length) from 21 complete specimens belonging to 14 species in order to establish a linear regression (Fig. 2).

This approach allows us to estimate allometric scaling of the radius in relation to TBL (see also Scheyer et al. 2014). That said, very few growth series can be measured for any given taxon; one of the few is Stenopterygius quadriscissus representing the best-sampled ichthyosaur species. This taxon is a parvipelvian ichthyosaur reaching a maximum size of 4 m, most certainly unrelated to the Welsh animal and therefore presumably with a different life history. Moreover, Stenopterygius and all thunnosaurians are characterized by a shortened tail compared to less derived ichthyosaurs (Maisch and Matzke 2000); for a given radius size, their TBL would therefore be shorter than for non-thunnosaurians. For these reasons, Stenopterygius is not the ideal candidate for comparison. Ideally, shastasaurids would provide the data for reliable estimates, but none of the numerous skeletons estimated at 15 m long is complete (Kosch 1990). Therefore, the best approach was to compile a linear relation based on as many taxa as possible, so as to cover the full spectrum of non-parvipelvian ichthyosaur body size and disparity. The correlated body measurements lie on lines with R2 values of 0.908 and 0.919 for radius width versus preflexural length and radius height versus preflexural length, respectively.

There may have been other large, possibly giant, ichthyosaurs in European Lower Jurassic deposits. Noteworthy are several bones of large size reported by McGowan (1996), including a large scapula (44.4 cm long) and a centrum (20.5 cm in diameter) both from Lyme Regis, Dorset, England, as well as a series of vertebrae from Banz, Germany (22 cm centrum in height). The largest complete skull reported from Lower Jurassic deposits so far is that of Temnodontosaurus platyodon, with a specimen (SMNS 50000) reported as more than 9 m long (McGowan 1996). However, our measurement of that SMNS 50000 specimen (see SOM, Supplementary Online Material available at  http://app.pan.pl/SOM/app60-Martin_etal_SOM.pdf) indicates this size has been overestimated (6 m instead). Other Lower Jurassic large forms from Europe include Temnodontosaurus azerguensis (Martin et al. 2012) and Leptonectes solei (McGowan 1993) with lengths of 8-10 m and 6 m, respectively. Temnodontosaurus platyodon may hold the record, with a skull length approaching 2 m (see measurements in McGowan 1996 for NHMUK PV R 51155 and BRLSI [BATGM] M3577) therefore surpassing the 1.5 m long skull specimens for which a TBL does not exceed 7 m (SMNS 50000). Nevertheless, with a possible maximal TBL of 10 m, Temnodontosaurus cannot be considered as gigantic and is smaller than the Welsh animal.

Although an estimate of TBL for NMW95.61G.1 is shorter than Shonisaurus sikanniensis (TBL estimate of 21 m; Nicholls and Manabe 2004), it is substantially longer than Cymbospondylus (less than 10 m TBL; Sander 1989; Merriam 1908) and is more comparable in size to Shonisaurus popularis from Nevada (Kosch 1990) or to the radius of Himalayasaurus tibetensis (Motani et al. 1999). This size certainly exceeds that of any known Lower Jurassic ichthyosaurs from Europe or elsewhere in the world.

Faunal turnover at the Triassic—Jurassic transition.—Nonparvipelvian ichthyosaur taxa have never been reported with confidence in strata younger than the middle Norian. Both the basal neoichthyosaurian Leptonectes and the basal thunnosaurian Ichthyosaurus from the “pre-planorbis Beds”, were previously considered as latest Rhaetian in age (Motani 2005), but a recent reassessment of the biostratigraphic marker of this unit regards it as basal-most Hettangian in age (Hillebrandt and Krystyn 2009; Benson et al. 2012). The basalmost parvipelvians Hudsonelpidia brevirostris and Macgowania janiceps are recorded much earlier, in lower and middle Norian strata, respectively, coexisting with shastasaurids (McGowan 1991, 1996, 1997; Jiang et al. 2006). The turnover between shastasaurid-dominated and parvipelvian-dominated ichthyosaur faunas, seemingly takes place in a stratigraphic interval with a poor fossil record: the upper Norian—Rhaetian interval. The present report indicates that a non-parvipelvian taxon occurs in lowermost Jurassic deposits, possibly representing one of the last members of the Shastasauridae. It substantially expands the stratigraphic range of non-parvipelvians and suggests that the replacement of the Triassic shastasaurids by the new parvipelvian radiation was more complex than previously appreciated and spread over several millions of years rather than being restricted to a single event, a view discussed by some authors (Benton 1986a, b; Mazin 1988; Bardet 1994, 1995). Consequently, further sampling is needed to precisely define the tempo and severity of this turnover. Notably, the lack of complete specimens hampers a full appraisal of the ecology of the previously last reported giant ichthyosaurs and the nature of their radiation/extinction. For instance, the predator Himalayasaurus tibetensis (TBL estimated over 10 m) from the Norian of Tibet (Motani et al. 1999) is known only from a fragmented skeleton and Shonisaurus sikanniensis Nicholls and Manabe, 2004, considered as a planktivorous animal is more complete, but poorly preserved. Only a fresh look at ichthyosaurian material from strata spanning the Triassic—Jurassic boundary, even based on fragmentary material, will help clarify the mechanisms of the end-Triassic ichthyosaurian faunal turnover.