SQUAMATA (LIZARDS AND SNAKES)

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 55-56

CROWN SQUAMATA (LIZARDS AND SNAKES) (52)

Node Calibrated. The clade that encompasses all modern lizards and snakes, their common ancestor, and all included taxa. There is dispute about the key diverging subclades within Squamata (see below).

Fossil Taxon and Specimen. Bharatagama rebbanensis Evans et al., 2002 , based on the holotype ( University of Jammu , Geology Department collections, VPL/JU/KR 66), a jaw bone.

Phylogenetic Justification. Bharatagama , represented by some 100 fragmentary specimens, shows numerous apomorphies of the acrodont Iguania: combination of a long fused angular; a short row of pleurodont anterior teeth in a shallow symphysial region; an elongate anteromedial symphysial surface restricted to the dorsal margin of the Meckelian fossa in adults; an acrodont dentition in which the teeth are broad but unflanged, and lack interstices; a pleuroacrodont additional series which follows a fully acrodont hatchling series; a strong pattern of precise dorsoventral (orthal) shear on the labial, but not lingual, surfaces of the dentary teeth; and an abutting premaxillarymaxillary contact in which a medial maxillary shelf extends behind an, apparently, narrow premaxilla ( Evans et al., 2002).

Minimum Age. 168.9 Ma

Soft Maximum Age. 209.5 Ma

Age Justification. Bharatagama and an unnamed pleurodont lizard taxon both come from the Upper Member of the Kota Formation, which is dated as Toarcian to?Aalenian (Bandyopadhyay and Sengupta, 2006), with an age in the range 182.7 Ma ± 0.7 Myr – 170.3 Ma ± 1.4 Myr ( Gradstein et al., 2012, p. 768).

The soft maximum date is set somewhat deeper in time because fossil deposits with extensive materials of small-scale diapsids are rare until we reach the Rhaetian (terminal Triassic), and those units include marine Rhaetic and terrestrially derived faunas in Europe and North America. The base of the Rhaetian is dated at 208-209 Ma by Muttoni et al. (2004, 2010), based on intercontinental comparisons of magnetostratigraphic records and radiometric dating, and 209.5 Ma by Gradstein et al. (2012, p. 716), and we select the oldest of these estimates.

Discussion. Squamata has been defined ( Evans, 2003, p. 517) as the clade that ‘accommodates the last common ancestor of the living Iguania and Scleroglossa and all of its descendants, and is diagnosed by a robust suite of derived character states’, including the specialized quadrate articulation with a dorsal joint (‘streptostyly’), loss of attachment between the quadrate and epipterygoid, subdivision of the primitive metotic fissure of the braincase, loss of the gastralia, emargination of the anterior margin of the scapulocoracoid, a hooked fifth metatarsal, and numerous soft tissue characters.

The choice of the subclades (Iguania and Scleroglossa) as definitional markers is potentially problematic because of differences in topology between morphological and molecular trees. Morphological cladograms (e.g., Estes et al., 1988; Lee, 1998; Evans, 2003) show a deep split of Squamata into two subclades, Iguania (= Chamaeoleonidae + Agamidae + Iguanidae ) and Scleroglossa, and so these can be selected as opposite poles of the clade. However, molecular phylogenies disagree, and they distinguish either Dibamidae ( Hedges and Vidal, 2009) , or Dibamidae + Gekkota (Wiens et al., 2012) as basal to all other lizard and snake clades. If the molecular trees are correct, then different pointers are required, or the definition can be apomorphy-based.

A second issue is that Squamata was formerly loosely divided into two sub groups, snakes (Serpentes) and lizards (Sauria), but only Serpentes is a clade (and even that is disputed by Wiens et al., 2012) that evolved from lizard ancestors, and so ‘Sauria’ is paraphyletic. Therefore, in determining the date of origin of Squamata, the age of the oldest fossil snake (currently an unnamed form from the Early Cretaceous [Rage and Richter, 1995]; Barremian, 130.8 Ma ± 0.5 Myr – 126.3 Ma ± 0.4 Myr; Gradstein et al., 2012, p. 838) is irrelevant.

In determining the oldest fossil squamate, we do not then follow the usual practice of pursuing a pair of lineages back to the node in question, but simply seek to identify the oldest fossil taxon with squamate synapomorphies. Evans (2003) reviews the fossil record in detail, and summarizes why numerous Triassic taxa formerly called ‘lizards’ of one sort or another are not: these all fall in other non-squamate and even non-lepidosaur clades, such as archosauromorphs, basal lepidosauromorphs, or even basal diapsids, and were often called ‘lizards’ only because they were small and diapsid.

Subsequent to Evans’ (2003) review, a supposed Triassic lizard, Tikiguania , was reported from the Late Triassic Tiki Formation of India by Datta and Ray (2006), but this has been rejected by Hutchinson et al. (2012) as almost certainly based on the mandible of a modern agamid that was mixed into surface sediments. This means that the oldest crown squamates are Jurassic, and several assemblages from the Early-Middle Jurassic of India, Britain, and Central Asia currently vie for the oldest spot, and of these those from the Kota Formation are potentially the oldest.