The new ichthyosauriformChaohusaurus brevifemoralis(Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China

Abstract

A new species of ichthyosauriform is recognized based on 20 specimens, including nearly complete skeletons, and named Chaohusaurus brevifemoralis. A part of the specimens was previously identified as Chaohusaurus chaoxianensis and is herein reassigned to the new species. The new species differs from existing species of Chaohusaurus in a suite of features, such as the bifurcation of the caudal peak neural spine and a short femur relative to trunk length. The specimens include both complete and partially disarticulated skulls, allowing rigorous scrutiny of cranial sutures. For example, the squamosal does not participate in the margin of the upper temporal fenestra despite previous interpretations. Also, the frontal unequivocally forms a part of the anterior margin of the upper temporal fenestra, forming the most medial part of the anterior terrace. The skull of the holotype largely retains three-dimensionality with the scleral rings approximately in situ, revealing that the eyeball was uncovered in two different directions, that is, laterally and slightly dorsally through the main part of the orbit, and dorsally through the medial extension of the orbit into the skull roof. This skull construction is likely a basal feature of Ichthyosauromorpha. Phylogenetic analyses place the new species as a sister taxon of Chaohusaurus chaoxianensis.

Repeated evolution of durophagy during ichthyosaur radiation after mass extinction indicated by hidden dentition

“…Ancestral character state reconstruction. We used a recently published phylogenetic hypothesis of Ichthyosauromorpha and its associated taxon-character matrix for ancestral state reconstruction 55 . The matrix reflects the dental morphology reported in the present study, as well as the latest findings on basal ichthyosauromorphs.…”

Cranial anatomy ofBesanosaurus leptorhynchusDal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications

“…12) as short on the basis of specimens PIMUZ T 4376 and BES SC 999, as their morphology is short compared with Cymbospondylus ( Motani, 1999 ). This morphological difference has been used as a character in recent phylogenetic analyses ( Ji et al, 2016 ; Huang et al, 2019 ).…”

“… Maisch & Matzke (1997a) described a separate coronoid in GPIT 1973/1, but we argue that a broken bone fragment was likely erroneously identified as the coronoid and that the coronoid in B. leptorhynchus was fused with the surangular, like in more derived ichthyosaurs. One specimen of the early-diverging ichthyopterygian Chaohusaurus brevifemoralis ( Huang et al, 2019 ) has a separate coronoid on one side of the mandible, and a coronoid fused with the surangular on the other side. The coronoid in that specimen forms a distinct, pointed process in the caudal portion of the mandible - the coronoid process.…”

“…To test the phylogenetic position of Besanosaurus leptorhynchus we used a modified matrix (see discussion) from Huang et al (2019) . This was analysed in TNT 1.5 ( Goloboff, Farris & Nixon, 2008 ; Goloboff & Catalano, 2016 ), with memory set to hold 99,999 trees.…”

“…As defined by Ji et al (2016) , Shastasauridae include six genera of long-bodied (presacral count > 55) forms: Shastasaurus , Besanosaurus , Guanlingsaurus , Guizhouichthyosaurus , Shonisaurus and ‘Callawayia ’ wolonggangense ( Camp, 1980 ; Dal Sasso & Pinna, 1996 ; Maisch & Matzke, 1997a ; Li & You, 2002 ; Nicholls & Manabe, 2004 ; Maisch et al, 2006b ; Pan, Jiang & Sun, 2006 ; Chen, Cheng & Sander, 2007 ; Shang & Li, 2009 ; Sander et al, 2011 ; Ji et al, 2013 ). The monophyly of Shastasauridae (recovered by e.g., Ji et al, 2013 , 2016 ; Jiang et al, 2016 ; Motani et al, 2017 ; Huang et al, 2019 ) has often been questioned and the clade has been recovered as paraphyletic by several authors (e.g., Maisch & Matzke, 2000 ; Sander, 2000 ; Sander et al, 2011 ; Moon, 2017 ; Moon & Stubbs, 2020 ). The validity of some of the shastasaurid taxa has also been questioned (e.g., Guanlingsaurus : Sander et al, 2011 ; Ji et al, 2013 ; Guizhouichthyosaurus : Shang & Li, 2009 ; ‘ Callawayia ’ wolonggangense : Chen, Cheng & Sander, 2007 ; Ji et al, 2016 ).…”