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| Archaeovaranus lii Dong, Wang, Zhao, Vasilyan, Wang & Evans, 2022. Illustration by Xiaocong Guo. |
Abstract
Monitor lizards (genus Varanus) are today distributed across Asia, Africa and Australasia and represent one of the most recognizable and successful lizard lineages. They include charismatic living species like the Komodo dragon of Indonesia and the even larger extinct Varanus prisca (Megalania) of Australia. The fossil record suggests that living varanids had their origins in a diverse assemblage of stem (varaniform) species known from the Late Cretaceous of China and Mongolia. However, determining the biogeographic origins of crown-varanids has proved problematic, with Asia, Africa and Australia each being proposed. The problem is complicated by the fragmentary nature of many attributed specimens, and the fact that the most widely accepted, and most complete, fossil of a stem-varanid, that of Saniwa ensidens, is from North America. In this paper, we describe a well-preserved skull and skeleton of a new genus of stem-varanid from the Eocene of China. Phylogenetic analysis places the new genus as the sister taxon of Varanus, suggesting that the transition from Cretaceous varaniform lizards to Varanus occurred in East Asia before the origin and dispersal of Varanus to other regions. The discovery of the new specimen thus fills an important gap in the fossil record of monitor lizards. The similar lengths of the fore- and hindlimbs in this new taxon are unusual among the total group Varanidae and suggest it may have had a different lifestyle, at least from the contemporaneous North American S. ensidens.
Keywords: China, evolution, early Eocene, Varanidae
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In the early Eocene, two adults of Archaeovaranus lii stand by a river among trees. The front lizard is searching for prey. In the front right is a Rhombomylus (a species of Glires). Illustration by Xiaocong Guo. |
Systematic palaeontology
Squamata Oppel, 1811
Anguimorpha Fürbringer, 1900
Varanidae Merrem, 1820
Archaeovaranus lii gen. et sp. nov.
Etymology. The generic name Archaeo- reflects the primitive phylogenetic position of this lizard in the evolution of Varanidae, and -varanus suggests the close relationship with the genus Varanus, whereas the specific name lii honours the late Prof. Chuankui Li for his important work on Palaeogene mammals, including some from the same locality as the holotype lizard described herein.
Holotype. IVPP V 22770, a nearly complete skeleton with an intact skull and associated but mostly disarticulated postcranial elements (figure 1a).
Locality and horizon. Dajian locality (IVPP fossil site catalogue number 76006, ...) (...), about 35 km northwest of Danjiangkou City, Hubei Province, China; Middle Member, Yuhuangding Formation (early Eocene; 52–56 Ma; the palaeomagnetic study suggesting a precise age of 53 Ma for the fossil-bearing horizon).
Diagnosis. The new genus and species is a medium-sized stem-varanid that resembles S. ensidens and Varanus and differs from helodermatids, palaeovaranids, lanthanotids and the Late Cretaceous stem-varanids in combining the following characters: posteriorly extended (retracted) external narial openings that reach frontals; a single median nasal; two lacrimal foramina, the ventral of which is fully enclosed by the lacrimal; elongated vomers; a U-shaped palatine; a short maxillary tooth row mostly confined to preorbital region, bearing well-spaced recurved teeth with basal plicidentine; intramandibular joint; splenial positioned anterior to level of dorsal prominence of the coronoid; anterior process of coronoid long; and distinct precondylar constriction on the vertebral centra. Archaeovaranus resembles S. ensidens and differs from Varanus in having cristae cranii of frontal separated in ventral midline, separate postfrontal and postorbital, a complete bony postorbital bar, teeth on palatine and pterygoid, posterolaterally placed basal tubera on basioccipital and a single coracoid emargination. It differs from S. ensidens in having a longer rostrum, two pterygoid tooth rows (versus a single row), and resembles Varanus but not Saniwa in having a dorsal lacrimal foramen enclosed by the prefrontal and lacrimal (rather than being restricted to the lacrimal). Archaeovaranus differs from both S. ensidens and known species of Varanus in having the forelimb and hindlimb, and the humerus and femur, of near equal length.
Liping Dong, Yuan-Qing Wang, Qi Zhao, Davit Vasilyan, Yuan Wang and Susan E. Evans. 2022. A New Stem-varanid Lizard (Reptilia, Squamata) from the early Eocene of China. Philosophical Transactions of the Royal Society B: Biological Sciences. 377(1847); ‘The impact of Chinese palaeontology on evolutionary research’. compiled and edited by Xiaoya Ma, Guangxu Wang and Min Wang. DOI: 10.1098/rstb.2021.0041
