Tuesday, May 24, 2016

[Paleontology • 2016] Sclerocormus parviceps • A Large Aberrant Stem Ichthyosauriform Indicating Early Rise and Demise of Ichthyosauromorphs in the Wake of the End-Permian Extinction

Sclerocormus parviceps 
Jiang, Motani, Huang, Tintori, Hu, Rieppel, Fraser, Ji, Kelley, Fu & Zhang, 2016

Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval. However, emerging discoveries of Early Triassic marine reptiles are questioning this traditional view. Here we present an aberrant basal ichthyosauriform with a hitherto unknown body design that suggests a fast radiation of early marine reptiles. The new species is larger than coeval marine reptiles and has an extremely small head and a long tail without a fluke. Its heavily-built body bears flattened and overlapping gastral elements reminiscent of hupehsuchians. A phylogenetic analysis places the new species at the base of ichthyosauriforms, as the sister taxon of Cartorhynchus with which it shares a short snout with rostrally extended nasals. It now appears that ichthyosauriforms evolved rapidly within the first one million years of their evolution, in the Spathian (Early Triassic), and their true diversity has yet to be fully uncovered. Early ichthyosauromorphs quickly became extinct near the Early-Middle Triassic boundary, during the last large environmental perturbation after the end-Permian extinction involving redox fluctuations, sea level changes and volcanism. Marine reptile faunas shifted from ichthyosauromorph-dominated to sauropterygian-dominated composition after the perturbation.

Systematc Paleontology
Reptilia Laurenti, 1768.
Diapsida Osborn, 1903.

Ichthyosauromorpha Motani et al., 2015.
Ichthyosauriformes Motani et al., 2015.

Nasorostra nov.

Etymology: Nasus (Latin nose) and rostrum (Latin beak), referring to the snout with the nasal bone reaching the tip.

Diagnosis: Rostrally elongate nasal reaching snout tip; preorbital and postorbital skull lengths sub-equal; frontal without distinctive posterolateral process; deep posterior mandible with slanting end and low jaw joint; ribcage deepest near shoulder; scapular blade wider distally than proximally.

Sclerocormus parviceps gen. et sp. nov.

Etymology: Genus name from Greek skleros and kormos, ‘stiff trunk’; species name from Latin parvus and caput, ‘small skull’.

Holotype: Anhui Geological Museum AGB6265.

Figure 1: The holotype of Sclerocormus parviceps gen. et sp. nov.
 (a) Whole specimen. (b) Skull. (c) Close-up of gastral basket. (d) Close-up of U-shaped haemal arches. (e) Right forelimb. (f) Shoulder elements. (g) Pelvic girdle and hind limb. (h) Skull elements.
Abbreviations: a, angular; ar, articular; as, astragalus; ca, calcaneum; car, caudal rib; ca.v, caudal vertebra; ce, centralia; cl, clavicle; d, dentary; dc, distal carpal; f, frontal; fe, femur; fi, fibula; he, hemal arch; il, ilium; in, intermedium; is, ischium; j, jugal, l, lacrimal; m, maxilla; mc, metacarpal; mt, metatarsal; n, nasal; p, parietal; pm, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; pu, pubis; q, quadrate; sa, surangular; sc, scapula; scl, scleral ossicles; sq, squamosal; sr, sacral rib; st, supratemporal; ti, tibia; u, ulna; ul, ulnare. Scale unit in (a) is 1 cm, other scale bars are 2 cm.
DOI:  10.1038/srep26232

 Diagnosis: Skull very short, occupying 6.25% of total length; tail long, about 58% of total length; body trunk short and deep; preorbital snout constricted and extremely short, about 30% of skull length; orbit large, more than one third of skull length; pineal foramen large, located at fronto-parietal suture; nasal large; ribs flattened, with blunt distal ends; gastralia robust, forming tight ventral basket; dorsal neural spines tall and vertical, with craniad and caudad flanges sandwiching the thickened shaft; caudal neural spines short with rounded top; femur straight, without shaft constriction.

Locality and horizon: From the first level of Majiashan Quarry, Chaohu, Anhui Province, China. Bed 719, about 27 m above the bottom of the Upper Member of the Nanlinghu Formation, within the ammonite Subcolumbites zone, Spathian, Olenekian, Lower Triassic (Fig. 2).

Da-Yong Jiang, Ryosuke Motani, Jian-Dong Huang, Andrea Tintori, Yuan-Chao Hu, Olivier Rieppel, Nicholas C. Fraser, Cheng Ji, Neil P. Kelley, Wan-Lu Fu and Rong Zhang. 2016. A Large Aberrant Stem Ichthyosauriform Indicating Early Rise and Demise of Ichthyosauromorphs in the Wake of the End-Permian Extinction. Scientific Reports. 6; 26232. DOI:  10.1038/srep26232
Ryosuke Motani, Da-Yong Jiang, Guan-Bao Chen, Andrea Tintori, Olivier Rieppel, Cheng Ji and Jian-Dong Huang. 2015. A Basal Ichthyosauriform with A Short Snout from the Lower Triassic of China. Nature. 517, 485–488. DOI: 10.1038/nature13866

Strange sea-dwelling reptile fossil hints at rapid evolution after mass ... http://bit.ly/1TJIby1 via @FieldMuseum @EurekAlertAAAS
Strange sea-dwelling reptile fossil hints at rapid evolution after mass extinction http://phy.so/383197123 via @physorg_com
Early Ichthyosaur Marks Marine Reptile Boom and Bust http://blogs.scientificamerican.com/laelaps/early-ichthyosaur-marks-earth-s-great-reptilian-comeback/