Revising Lizard Evolution in Gondwana

The Squamata (lizards and snakes), might be the most diverse and specious of all the extant reptiles but their evolutionary history remains a mystery.  The preservation potential of these reptiles can vary dependent on the characteristics of the ancient ecosystems which they inhabited.  In addition, the small size of many of the early species of lizard and snake makes finding fossilised bones and other clues as to the history of this Order all the more difficult.

However, a team of scientists including Tiago R. Simões (Dept of Biological Science, University of Alberta), have published a paper in the journal “Nature Communications” that sheds light on the evolution of the iguanas and their near relatives.

Studying the Iguanians

The family of extant reptiles known as the iguanians (iguanas and agamids) are one of the most diverse and widespread type of lizard today.  However, like other types of lizard, their evolutionary origins are uncertain.  There are two main divisions within this family, firstly there is a sub-group called the acrodonts, these lizards have no root or socket to their teeth and the teeth are attached to the top of the jaw bones.  In the other sub-group, the pleurodonts they do not have roots on their teeth either, but instead the teeth attach to the inside portion of the jaw.

Acrodont iguanians are confined to the Old World, while pleurodont iguanians are found only in the Americas.  A newly described fossil however, breaks this pattern.  The international team of scientists have published a paper on a Late Cretaceous acrodont iguanian, the oldest known New World acrodont iguanian.

An Illustration of the First New World Acrodont Iguanian – G. sulamericana

The fossils date from around 80 million years ago.

The fossils date from around 80 million years ago.

Picture credit: Julius Csotonyi

Gueragama sulamericana

The fossils, including the holotype material, a partial lower jaw come from the Goio-Erê Formation, exposed near Cruzeiro do Oeste, in Paraná State, southern Brazil.  Fossils found in this locality represent an arid, very dry environment dominated by pterosaurs and large dinosaurs.  However, scurrying around the hot, desert-like environment was a short, rather stubby lizard.  It has been named Gueragama sulamericana, which translates from the local dialect and Portuguese as “ancient agama from South America”.

The discovery of these fossils of an ancient, New World acrodont means that in the distant past, lizards with the acrodont dentition had a worldwide distribution and were very probably widespread across the ancient landmass of Gondwana.

The Holotype Jaw Fossil (Various Views)

An ancient toothy lizard.

An ancient toothy lizard.

Picture credit: Universidade do Contestado, Santa Catarina, Brazil.

Scale bar in the picture = 2 mm.

Fossil Discovery Raises Questions

A number of questions have been raised with this discovery, for example, if only the pleurodont iguanians are found in the New World today, then what happened to the acrodonts that once lived in this part of the world too?

Lead author of the scientific paper, Tiago R. Simões stated:

“It becomes clear acrodontan iguanians migrated throughout the southern continents much earlier than previously thought (actually reaching regions where they do not inhabit today) by the order of tens of millions of years.”

This new research extends the fossil diversity of Late Cretaceous Brazil and suggests that the arid deserts of this part of the world may have supported a greater range of lizards than previously thought.  The scientists hope to find more lizard fossils, including material representing Gueragama so that they will be better able to understand the evolution and radiation of the stem acrodontans.

As for the date given to the G. sulamericana fossil material, we at Everything Dinosaur estimate these fossils to be more than 80 million years of age.  The strata which makes up this part of the Goio-Erê Formation is believed to date from the Turonian to the early Campanian faunal stages of the Late Cretaceous (90-80 million years ago, approximately).

Late Cretaceous South America

South America in the Late Cretaceous

South America in the Late Cretaceous

Picture credit: Nature Communications

Fossil Finds in South America

Between the Aptian/Albian faunal stages and the Campanian faunal stage of the Cretaceous, sphenodontians were thought to be the dominate animals filling the iguanian niche.  Sphenodontian fossils have been found in northern Patagonia, in the provinces of Chubut (Tres Cerros), Río Negro (Los Alamitos, Cerro Tortuga, Cerro Bonaparte and La Buitrera) and Neuquén (El Chocón), these discoveries are represented by black circles on the map above.

Lizards were present in the state of Ceará in north-eastern Brazil (Araripe Basin), as well as in the south-eastern/southern states of Minas Gerais (Peirópolis), São Paulo (Marília and Presidente Prudente) and Paraná (Cruzeiro do Oeste), and in the province of Río Negro (Cinco Saltos and La Buitrera), Argentina, represented by yellow stars on the map. The red star indicates the type locality of G. sulamericana in southern Brazil.

Recently, Everything Dinosaur reported on another Brazilian Squamata fossil discovery.  Spotted by chance on a tour of a German museum, scientists have identified the first example of a four-footed snake, an animal believed to be a transitional creature between limbed lizards and true snakes.

To read this article: Fossil Snake with Four Limbs Described.