The Australian rainforest dragon (Hypsilurus spinipes) does not belong to the same branch of the lizard family tree as Tikiguania, but does share the same general appearance, say the researchers.
Credit: Mark Hutchinson
Skull of an Australian bearded dragon (Pogona barbata) showing the interlocking teeth that are typical of modern agamids (‘dragon lizards’).
Credit: Mark Hutchinson
SYDNEY: What was widely thought to be the oldest known lizard turns out to be a species of modern lizard related to the Australian dragon lizard.
Since 2006, a fossilised reptile found in India called Tikiguania estesi has been labelled the earliest known lizard, thought to have existed during the Triassic Period, around 220 million years ago. But new research has reviewed the original description of the species and found that it actually belongs to a group of advanced lizards that occupy southeast Asia and India today.
"The fossil looked incredibly familiar looking, and in too beautiful condition for something that was supposed to be so old," said herpetologist Mark Hutchinson of the South Australian Museum in Adelaide and lead author of the study published in Biology Letters today.
A Triassic fossil?
According to Hutchinson, the original researchers from India examined Tikiguania estesi and classified it based purely on their observations of the lone left jaw. The jaw was found in soft mudstone in central India within a rock layer that contained specimens dated to the Triassic Period. The researchers said that the specimen was quite distinct from other species of extinct lizards and concluded that they had found a new species of Triassic reptile.
"What they didn't do was compare the fossil to any members of the living lizard fauna," said Hutchinson. He said that the Triassic deposit from which the jaw bone was found has been weathered on the surface over the past million years, and that much more 'modern' specimens could have been entombed in the rock.
"It is not unusual in sites like this where you have easily eroded, superficial deposits, and that you will get recent animals that have fallen into that old sediments, as well as genuine ancient fossils that are from the undisturbed layers," he said.
Related to dragon lizards
Hutchinson's study compared the jawbone of Tikiguania estesi with both the skeletons and the DNA of living lizards as well as ancient lizard fossils. The comparisons were used to determine the evolutionary relationships of Tikiguania estesi and found that the specimen belonged to a modern subgroup of dragon lizards that live in India.
"If this [fossil] was from the Triassic it would be very general, ancestral and basic in form. But it's not. It's a member of a group that happens to be unusually advanced and sophisticated in its anatomy. Something as modern as that is most unlikely to be 220 million years old," said Hutchinson.
Setting the record straight
Several recent studies of lizard evolution have used Tikiguania estesi to mark the appearance of the first unambiguous lizards in the fossil record, causing them to draw misleading conclusions about the timing and the structure of the earliest stage of lizard evolution, according to Hutchinson.
"We can't categorically prove that it [Tikiguania estesi] is not Triassic, but there is all the evidence that suggests it is extremely modern," said Hutchinson. "We didn't find a specimen of living fauna that exactly matched it, but [the research] is enough to show with a high degree of confidence that it belongs to this modern advanced group of lizards - it's not in any sense a deep ancestral primitive creature."
Paleaeontologist Robert Reisz of the University of Toronto Mississauga in Canada commented on the research saying that is "interesting and laudable" and commended the researchers for not "blindingly following the data" and providing a "more reasonable conclusion" that the isolated find is an anomaly.
Hutchinson hopes to further connect working with fossils and living fauna, so a better interpretation of these few specimens can be placed as reliably as possible with their nearest relatives.
