Small fossil reptile could help to explain large evolutionary mystery

A remarkably complete fossil discovered in the USA is revealing more about a once widespread group of reptiles known as the rhynchocephalians.

Today, New Zealand's tuatara is the only living species of these lizard-like animals, but between about 250 and 66 million years ago there were more than 40 species living around the world. One of these was Opisthiamimus gregori, which would have measured around 16 centimetres long and eaten insects and possibly even snails.

This new species may be the first of a number of previously undescribed rhynchocephalians from the Morrison Formation that could offer an insight into how these animals would have lived.

Dr Dave DeMar, the lead author of the description of this species, says, 'O. gregori is so complete that it warrants attention, telling us a lot more about these critters than we could have hoped for.'

'Three other rhynchocephalians have previously been described from this area, but their holotypes (the original name-bearing specimen of each species) are all isolated jawbones with teeth.'

'For O. gregori to be this complete is magnificent. There are other specimens collected from the same rocks which have yet to be described and will provide even deeper knowledge of these ancient reptiles.'

The findings of the study, co-authored by Matthew T. Carrano and Museum fossil reptile curator Marc Jones, were published in the Journal of Systematic Palaeontology.

What are rhynchocephalians?

While superficially they look similar, rhynchocephalians are not lizards. Instead, they are the closest relatives of squamates, the group which includes snakes, lizards and limbless animals known as amphisbaenians.

Both groups are believed to have diverged during the Late Permian or Early Triassic around the time of the Permian-Triassic mass extinction - the most significant in history. As life recovered and diversified during the Triassic, the rhynchocephalians took up a variety of roles as they spread globally.

'The heyday of the rhynchocephalians was during the Mesozoic,' Dave says. 'They lived alongside the dinosaurs and had high numbers of species as well as a diverse range of body shapes and lifestyles.'

'This included large herbivores, insectivores and even aquatic forms with very elongated bodies and short limbs.'

However, following the end of the Jurassic around 145 million years ago, their numbers began to dwindle. By the time the Mesozoic came to an end, only a handful of species were known, all living in the southern hemisphere. Today, the tuatara is all that is left.

'It's not certain why the rhynchocephalians became less diverse during the Cretaceous as the fossil record is quite incomplete for these animals,' Dave explains. 'Their decline appears to correlate with increasing numbers of squamate species, suggesting competition could have played a role.'

'The end of the Jurassic also sees the increasing fragmentation of Pangea, which would have caused changes in the climate and habitat the rhynchocephalians lived in.'

Discovering more specimens of new and existing species will fill gaps in the fossil record and allow researchers to gain a clearer understanding of what happened to these once-predominant animals.

The extraordinary preservation of O. gregori provides an unprecedented insight on the twilight years of the rhynchocephalians' reign during the Late Jurassic.

How was Opisthiamimus gregori discovered?

The new species was uncovered in the Morrison Formation, a Late Jurassic rock formation most well-known for the discovery of dinosaurs such as Allosaurus and Diplodocus.

Work on these dinosaurs has overshadow the smaller ancient reptiles that would have lived alongside them, but researchers are now beginning to find out more about these rhynchocephalians.

The holotype of O. gregori was discovered in 2010 by Smithsonian fossil preparator Pete Kroehler, who saw bones emerging from an excavated block of stone.

These were later revealed to be the jaw bones after Joseph Gregor, a volunteer at the Smithsonian, spent countless hours chiselling away the rock to reveal them. His work was recognised with the new species being named in his honour.

The skeleton is almost complete, except for parts of the tail and hind limbs, which makes a change from the generally fragmentary remains of other specimens. Fossil rhynchocephalians are generally only represented by incomplete jaws with teeth, making the completeness of this fossil and a handful of other species particularly important.

While some of the bones were crushed and distorted by the process of fossilisation, the researchers were able to use micro-CT scans to reconstruct how the species would have looked. By comparing the teeth and body size with other species, they can get an idea about the kind of ecological role O. gregori would have played in its community.

'Whereas Opisthiamimus gregori probably ate animals with hard shells, other rhynchocephalians such as Opisthias rarus and Eilenodon robustus may have been able to eat small vertebrates and plant matter respectively,' Dave says. 'This dietary diversity would have allowed them to inhabit similar environments without competing directly with one another.'

Researchers are now hoping to find out more about this Jurassic ecosystem by describing more species based on published and unpublished specimens. As more species are discovered, this may finally resolve why the rhynchocephalians declined so sharply from the Cretaceous until today.