Tuesday, May 24, 2016

[Paleontology • 2014] Cartorhynchus lenticarpus • A Basal Ichthyosauriform with A Short Snout from the Lower Triassic of China

Cartorhynchus lenticarpus 
Motani, Jiang, Chen, Tintori, Rieppel, Ji & Huang, 2014

10.1038/nature13866 paleoArt 
by Vitaliy Melnik  malvit.deviantart.com 


 The incompleteness of the fossil record obscures the origin of many of the more derived clades of vertebrates. One such group is the Ichthyopterygia, a clade of obligatory marine reptiles that appeared in the Early Triassic epoch, without any known intermediates. Here we describe a basal ichthyosauriform from the upper Lower Triassic (about 248 million years ago) of China, whose primitive skeleton indicates possible amphibious habits. It is smaller than ichthyopterygians and had unusually large flippers that probably allowed limited terrestrial locomotion. It also retained characteristics of terrestrial diapsid reptiles, including a short snout and body trunk. Unlike more derived ichthyosauriforms, it was probably a suction feeder. The new species supports the sister-group relationships between ichthyosauriforms and Hupehsuchia, the two forming the Ichthyosauromorpha. Basal ichthyosauromorphs are known exclusively from south China, suggesting that the clade originated in the region, which formed a warm and humid tropical archipelago in the Early Triassic. The oldest unequivocal record of a sauropterygian is also from the same stratigraphic unit of the region.

Reptilia Laurenti, 1768
Diapsida Osborn, 1903

Ichthyosauromorpha nov.

Diagnosis. The last common ancestor of Ichthyosaurus communis and Hupehsuchus nanchangensis, and all its descendants. Anterior flanges on humerus and radius present; ulna distal width equal to or greater than proximal width; forelimb longer than or almost equal to hindlimb; manus length at least about three-quarters the length of the stylopodium and zeugopodium combined; fibula extending further post-axially than femur; transverse process of neural arch extremely short or absent.

Ichthyosauriformes nov.

Diagnosis. All ichthyosauromorphs more closely related to Ichthyosaurus communis than Hupehsuchus nanchangensis. Nasal extending anteriorly, well beyond external naris; scleral ring large, filling orbit; snout constricted in dorsal view; converging digits with limited interdigital space.

Cartorhynchus lenticarpus gen. et sp. nov.

Etymology. καρτοσ (Greek), meaning shortened; ρηψνχηοσ (Greek), meaning snout; lentus (Latin), meaning flexible; carpus (Latin), meaning wrist. Named after truncated snout and cartilaginous wrist.

Holotype. Anhui Geological Museum AGB6257.

Locality and horizon. From the second level of Majiashan Quarry (31° 37′ 26″ N, 117° 49′ 19″ E), near downtown Chaohu, Hefei City, Anhui Province, China (Fig. 1a). Bed 633, about 13 m above the bottom of the Upper Member of the Nanlinghu Formation (Fig. 1b), within the ammonite Subcolumbites zone, Spathian, Olenekian, Lower Triassic (Fig. 1b).

Figure 2: The holotype of Cartorhynchus lenticarpus gen. et sp. nov.
a, Whole specimen. b, Close-up of ribs. c, Ribs of Chaohusaurus (AGM CH-628-16) for comparison. d, Skull and shoulder elements. e, Skull of a newborn Chaohusaurus20 drawn to the same scale as d for comparison. f, Right forelimb. g, Right hindlimb.
a, angular; as, astragalus; ca, calcaneum; cbi, first ceratobranchial; cl, clavicle; d, dentary; f, frontal; Fe, femur; Fi, fibula; H, humerus; i, intermedium; icl, interclavicle; j, jugal; l, lacrimal; m, maxilla; n, nasal; op, opisthotic; p, parietal; pm, prefrontal; po, postorbital; prf, prefrontal; ptf, postfrontal; q, quadrate; qj, quadratojugal; R, radius; r, radiale; sa, surangular; sc, scapula; scl, scleral ossicles; sp, splenial; sq, squamosal; st, supratemporal; Ti, tibia; U, ulna; u, ulnare; i–v (in g), metapodials. Scale bars, 1 cm.  DOI: 10.1038/nature13866 

Diagnosis. Autapomorphies are: snout only half as long as the rest of the skull; very large hyoid; forelimb strongly curved posteriorly; anteriorly curved hindlimb; ribs robust, with proximal intercostal space narrower than ribs; scapula wider distally than proximally; autopodium with broadly spaced tiny ossifications; only three manual digits ossified; gastralia without median element. Other features: mandible deep; trunk shorter than in ichthyopterygians by at least five vertebrae; pineal foramen very large; interclavicle cruciform; parapophyses confluent with anterior vertebral margin.

Cartorhynchus is the smallest ichthyosauriform to date. The preserved length of the specimen is 21.4 cm (Fig. 2a). Total body length is estimated to be about 40 cm, assuming tail proportions of basal ichthyopterygians. Of the 31 pre-sacral vertebrae, 5 seem to be cervical. Ichthyopterygians typically have an elongated body with 40–80 pre-sacral vertebrae, except for Chaohusaurus, which has about 36 (Fig. 3a). The pre-sacral vertebral count of extant terrestrial reptiles with well-developed limbs ranges from 16 to 36, with 24 being the norm10. Cartorhynchus is within this terrestrial range (Fig. 3a).

Ryosuke Motani, Da-Yong Jiang, Guan-Bao Chen, Andrea Tintori, Olivier Rieppel, Cheng Ji and Jian-Dong Huang. 2014. A Basal Ichthyosauriform with A Short Snout from the Lower Triassic of China. Nature. 517, 485–488. DOI: 10.1038/nature13866

Short-snouted, suction-feeding ‘proto-ichthyosaur’ sheds light on fish-lizard beginnings | Tetrapod Zoology, Scientific American Blog Network http://blogs.scientificamerican.com/tetrapod-zoology/2014/11/12/cartorhynchus-and-fish-lizard-beginnings/

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