A revision of the Latipinnate ichthyosaurs of the Lower Jurassic of England (Reptilia, Ichthyosauria)

Abstract

A study of 60 latipinnate ichthyosaurs from the English Lower Lias (Lower Jurassic) reveals only four valid species; Ichthyosaurus communis Conybeare, /. breviceps Owen, /. conybeari Lydekker, and /. tenuirostris Conybeare. Emended diagnoses and descriptions are given. /. communis, a medium-sized ichthyosaur reaching a maximum length of about 2.5 m, is the commonest species. /. tenuirostris, of similar size to /. communis, is less common and characterized by extreme snout length. /. breviceps, a small species with a maximum recorded body length of 1.3 m, is scarce and is characterized by an abbreviated snout. The rarest latipinnate is /. conybeari, with only two individuals known. Both are small, with an estimated body length of 1.3 m in the larger individual. The snout is long, slender, and sharply pointed, and the orbit appears to be relatively large. Zusammenfassung Eine Studie iiber 60 latipinnate Ichthyosaurier aus dem englischen unteren Lias (Unterer Jura) ergibt nur vier giiltige Arten; Ichthyosaurus communis Conybeare, /. breviceps Owen, /. conybeari Lydekker, /. tenuirostris Conybeare. Verbesserte Diagnosen und Beschreibungen werden gegeben. /. communis, die gewohnlichste Art, ist ein mittelgrosser Ichthyosaurier, welcher eine Lange von ungefahr 2.5 Meter erreicht. /. tenuirostris, von ungefahr derselben Grosse wie /. communis, ist seltener und zeichnet sich durch seine ungewohnlich lange Schnauze aus. /. breviceps, eine kleine Art, mit einer Lange bis zu 1.3 Meter, ist sehr selten und zeichnet sich durch seine verkiirzte Schnauze aus. Die seltenste Art ist /. conybeari, von welcher nur zwei Exemplare bekannt sind. Beide sind klein, das grossere Exemplar hat eine abgeschatzte Lange von 1.3 Meter. Die Schnauze ist lang, schmal, scharf zugespitzt, und die Augenhohlen scheinen verhaltnismassig gross zu sein. /. tenuirostris Conybeare. Results of the statistical analyses are shown in Tables 1 and 2, and their size ranges are shown in Fig. 1.

A large skull of Ichthyosaurus (Reptilia: Ichthyosauria) from the Lower Sinemurian (Lower Jurassic) of Frick (NW Switzerland)

“…Of these, only one, Ichthyosaurus communis, is known to reach a skull length in excess of 500 mm. the present specimen shows numerous diagnostic characters of I. communis, as the species was most recently diagnosed by Maisch & Matzke (2000a, b): it is large, reaching more than 600 mm in skull length, it has a moderately large orbit (as contrasted to the very large orbits of I. breviceps and I. conybeari; see Mcgowan [1974] for more details), its teeth are robust and curved, with blunt tips, their roots are never abruptly expanding (in contrast to I. intermedius; see Maisch [1997]), they have in part a somewhat rectangular cross-section (godefroit 1996) and the lacrimal has an enormous anterior extension below the premaxilla (see Maisch & Matzke 2000b). some other diagnostic features, such as the morphology of the jugal, quadratojugal and the number of maxillary teeth can, of course, not be assessed in the present specimen due to incomplete preservation.…”

“…-Large species, skull reaching more than 600 mm in length, robust and moderately long snout (ratio length of snout to length of jaw 0.57-0.69), moderately large orbit (ratio of orbit length to jaw length 0.20-0.28, but usually < 0.25), less than 20 maxillary teeth, robust and curved teeth, with blunt tips, roots never abruptly expanding, lacrimal with enormous anterior extension below premaxilla, jugal with rounded cross-section, quadratojugal moderately short dorsoventrally, reaching down distinctly below two thirds the orbital height, quadrate ascending plate robust, not a thin lamina as in Ichthyosaurus intermedius (modified from Mcgowan 1974 andMatzke 2000a).…”

“…the genus Ichthyosaurus is represented by three (Mcgowan 1974) or four (Maisch 1997;godefroit 1996;Maisch & Matzke 2000a) species from the early Early Jurassic. Of these, only one, Ichthyosaurus communis, is known to reach a skull length in excess of 500 mm.…”

Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary

“…Although the Jurassic eurhinosaurid Leptonectes and thunnosaurid Ichthyosaurus are both reported from the latest Triassic in the pre-planorbis beds of the Rhaetian (16,17), this is not evidence for their survival through the crisis, as these Triassic units fall within the ∼100,000 y after the mass extinction horizon and before the Tr-J boundary (3).…”

Ichthyosaurs from the Jurassic of Skye, Scotland

“…The lack of carinae in the Skye specimens differs from T. eurycephalus, T. trigonodon and T. platyodon (Godefroit 1993;Martin et al 2012), although this feature appears variable in the latter. The root is not swollen, unlike in Temnodontosaurus species for which teeth are known (although this is variable, ranging from being only slightly swollen to bulbous, as in T. eurycephalus) (McGowan 1974(McGowan , 1994 Bennett et al 2012). We can exclude both Skye specimens from I. intermedius (considered by McGowan & Motani 2003 to be a subjective junior synonym of I. communis), as this taxon has 'waisted' dentition, namely a narrow crown and an inflated root, which gives the crown-root junction a 'stepped' appearance (Maisch & Matzke 2000).…”