The remains of the earliest dinosaurs may lie undiscovered in the Amazon and other equatorial regions of South America and Africa, suggests a new study led by UCL (University College London) researchers.
Currently, the oldest known dinosaur fossils date back about 230 million years and were unearthed further south in places including Brazil, Argentina and Zimbabwe. But the differences between these fossils suggest dinosaurs had already been evolving for some time, pointing to an origin millions of years earlier.
The new study, published in the journal Current Biology, accounted for gaps in the fossil record and concluded that the earliest dinosaurs likely emerged in a hot equatorial region in what was then the supercontinent Gondwana – an area of land that encompasses the Amazon, Congo basin, and Sahara Desert today.
Lead author and PhD student Joel Heath (UCL Earth Sciences and the Natural History Museum, London) said: "Dinosaurs are well studied but we still don't really know where they came from. The fossil record has such large gaps that it can't be taken at face value.
"Our modelling suggests that the earliest dinosaurs might have originated in western, low-latitude Gondwana. This is a hotter and drier environment than previously thought, made up of desert- and savannah-like areas.
"So far, no dinosaur fossils have been found in the regions of Africa and South America that once formed this part of Gondwana. However, this might be because researchers haven't stumbled across the right rocks yet, due to a mix of inaccessibility and a relative lack of research efforts in these areas."
The modelling study drew on fossils and evolutionary trees of dinosaurs and their close reptile relatives, as well as the geography of the period. It accounted for gaps in the fossil record by treating areas of the globe where no fossils had been found as missing information rather than areas where no fossils exist.
Initially, early dinosaurs were vastly outnumbered by their reptile cousins.
These included the ancestors of crocodiles, the pseudosuchians (an abundant group including enormous species up to 10 metres long), and pterosaurs, the first animals to evolve powered flight (flying by flapping wings rather than gliding), who grew as big as fighter jets.
By contrast, the earliest dinosaurs were much smaller than their descendants – more the size of a chicken or dog than a Diplodocus. They walked on two legs (were bipedal) and most are thought to have been omnivores.
Dinosaurs became dominant after volcanic eruptions wiped out many of their reptile relatives 201 million years ago.
The new modelling results suggested that dinosaurs as well as other reptiles may have originated in low-latitude Gondwana, before radiating outwards, spreading to southern Gondwana and to Laurasia, the adjacent northern supercontinent that later split into Europe, Asia and North America.
Support for this origin comes from the fact it is a midpoint between where the earliest dinosaurs have been found in southern Gondwana and where the fossils of many of their close relatives have been discovered to the north in Laurasia.
As there is uncertainty about how the most ancient dinosaurs were related to one another and to their close relatives, the researchers ran their model on three proposed evolutionary trees.
They found strongest support for a low-latitude Gondwanan origin of the dinosaurs in the model that counted silesaurids, traditionally regarded as cousins of dinosaurs but not dinosaurs themselves, as ancestors of ornithischian dinosaurs.
Ornithischians, one of the three main dinosaur groups that later included plant eaters Stegosaurus and Triceratops, are mysteriously absent from the fossil record of these early years of the dinosaur era. If silesaurids are the ancestors of ornithischians, this helps to fill in this gap in the evolutionary tree.
Senior author Professor Philip Mannion (UCL Earth Sciences) said: "Our results suggest early dinosaurs may have been well adapted to hot and arid environments. Out of the three main dinosaur groups, one group, sauropods, which includes the Brontosaurus and the Diplodocus, seemed to retain their preference for a warm climate, keeping to Earth's lower latitudes.
"Evidence suggests the other two groups, theropods and ornithischians, may have developed the ability to generate their own body heat some millions of years later in the Jurassic period, allowing them to thrive in colder regions, including the poles."
The earliest known dinosaurs include Eoraptor, Herrerasaurus, Coelophysis, and Eodromaeus.
