Bharatagama

Bharatagama; Temporal range:Early Jurassic,190Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Lepidosauria
Genus:	†Bharatagama; Evanset al.,2002
Type species
	†Bharatagama rebbanensis; Evanset al.,2002

Bharatagamais an extinctgenusoflepidosaurfrom theEarly Jurassicof India. It has been suggested to be one of the oldest known lizards and the oldest knowniguanian.^[1]Thetypeand only species isBharatagama rebbanensis,named in 2002. Over one hundred fossils ofBharatagamahave been found in theKota Formation,which outcrops in thePranhita–Godavari Basinand dates back to about 190 million years ago (Ma). Despite its abundance,Bharatagamais known only from isolated jaw bones mixed together in microvertebrate assemblages with equally fragmentary remains of fish,sphenodontians,dinosaurs,crocodylomorphs,and mammals. These fossils represent all stages of development, from hatchlings to adults. The total length of the skull in adult specimens is estimated to have been about 15 millimetres (0.59 in).^[2]Later analysis suggested that the taxon might be a member ofRhynchocephalia.^[3]

Description and relationships

Bharatagamahas been suggested to belong a group of iguanians calledAcrodonta,which today includeschameleonsandagamids.Modern acrodontans are characterized by their acrodont dentition, meaning that their teeth implant along the margins of the jaws rather than their inner surfaces, the so-called pleurodont dentition seen in most other lizards. Most of the teeth in the jaws ofBharatagamaare acrodont, but the first five pairs in the lower jaw and first four in the upper jaw are pleurodont. These teeth are enlarged, recurved, and striated. The acrodont teeth behind them are sharp and triangular, likely adapted for shearing food. Some specimens preserve hatchling teeth between the acrodont and pleurodont teeth, which are conical and laterally compressed.^[2]

Sphenodontians are also abundant in the Kota microvertebrate assemblage and have acrodont dentitions, raising the possibility thatBharatagamais not one of the earliest lizards but rather a sphenodontian misidentified as a lizard.Bharatagamaalso shares with sphenodontians an enlargedcoronoid processon the lower jaw. However, this feature is also seen in some acrodontans and is not unique to Sphenodontia. The combination of features such as pleurodont teeth at the front of the jaws, hatchling teeth behind them, the lack of wear marks on the inner surfaces of the teeth (a characteristic feature of sphenodontians), and the shape of themaxilla,premaxilla,andangular boneswere suggested to be evidenceBharatagamabeing an early acrodont lizard.^[2]However, a later 2018 study suggested that the taxon had more in common with rhynchocephalians that squamates, and that it had no unambiguous synampomorphies that would warrant it being placed in Squamata.^[3]

Biogeography and evolutionary context

Aside fromBharatagama,the oldest known lizards are from circa-165 MaMiddle Jurassicdeposits in England. These lizards are all members ofScleroglossa,a largecladeor evolutionary grouping hypothesized to include all non-iguanian lizards. The presence ofBharatagama190 million years ago in India provides evidence that the basal split between Iguania and Scleroglossa occurred around this time, and that the earliest iguanians underwent anevolutionary radiationin the southern supercontinentGondwana.The paucity of early Mesozoic microvertebrate assemblages in regions formerly part of Gondwana may explain the large time gap betweenBharatagamaand Cretaceous iguanians, which lived in the northern supercontinentLaurasiaand have a good fossil record in North America, Europe, and Asia.^[2]