By using dinosaurs' close reptilian cousins to close gaps in the fossil record, a new model points to an ancient supercontinent as the birthplace of the extinct creatures.
(CN) -- Clues to the mystery of dinosaurs' origin story are likely still buried with the oldest members of their kind, which a new study suggests may lie undiscovered along the equator in South America and Africa.
Today's oldest known fossils were unearthed in areas like Zimbabwe, Argentina and Brazil and date back about 230 million years. However, the differences among those remains suggest dinosaurs had already long been evolving by that time, leading researchers to believe there are far earlier specimens to uncover.
The first dinosaurs could have emerged on the Gondwana supercontinent millions of years before today's oldest-known dinosaurs, researchers from University College London write in a new study published Thursday in the journal Current Biology. That ancient landmass encompasses the modern-day Amazon, Sahara Desert and Congo basin.
"Dinosaurs are well studied but we still don't really know where they came from," lead author and Ph.D. student Joel Heath said in a statement. "The fossil record has such large gaps that it can't be taken at face value."
Heath and the research team used modeling to pinpoint where dinosaurs may have originated.
"This is a hotter and drier environment than previously thought, made up of desert- and savannah-like areas," Heath said of Gondwana.
The researchers analyzed fossils, the geography of the time period and the evolutionary trees of dinosaurs and closely related reptiles to model possible biogeographic origin points.
"So far, no dinosaur fossils have been found in the regions of Africa and South America that once formed this part of Gondwana," Heath said. "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."
It's also possible that the region lacked suitable rocks and climate for preserving skeletal fossils, but the presence of fossils in regions with similar conditions casts doubt on that notion, the authors wrote.
The gaps in the fossil record may be why previous studies have not fingered the low-latitude region of Gondwana as a potential origin point. To account for those gaps, the researchers treated areas where no fossils have been found as simply missing information -- rather than assuming no fossils exist there.
According to the model, it's likely that dinosaurs and other reptiles moved outward into southern Gondwana and north to Laurasia, the supercontinent that eventually splintered into Europe, Asia and North America. Low-latitude Gondwana is a good candidate because it marks a midway point between finds of the earliest known dinosaurs and their close relatives.
Reptiles initially outnumbered early dinosaurs until volcanic eruptions around 201 million years ago turned the tables.
The researchers ran the model on three proposed evolutionary trees to account for uncertainty about early dinosaurs' relationships to one another, and to their close relatives.
The model provided the strongest support for low-latitude Gondwana as dinosaurs' origin when assuming their relatives the silesaurids were ancestors of ornithischian dinosaurs -- one of three main types of dinosaurs that came to include herbivores Triceratops and Stegosaurus. Linking those two groups link could help fill in missing pieces of the fossil record, researchers said, since the early dinosaur era does not include ornithischian fossils.
"Our results suggest early dinosaurs may have been well adapted to hot and arid environments," senior author professor Philip Mannion said in a statement. "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."
The two other groups, therapods and ornithischians, likely evolved to generate their own body heat millions of years later in the Jurassic period, Mannion said, allowing them to live in colder regions.
The researchers suggest further sampling in Triassic Gondwana could lead to more information.