Australian Museum’s Professor Tim Flannery and colleagues argue that the ancestors of Theria (placental and marsupial mammals) evolved in the supercontinent Gondwana 50 million years before migrating to Asia during the Early Cretaceous epoch, some 126 million years ago.
An artist’s impression of an early mammal-like animal. Image credit: Anatomical Society / Wiley.
“For almost 200 years it has been believed that the placental mammals, and the related marsupials, had originated in the northern hemisphere, as that is where the majority of mammal diversity is now found, and where the most abundant fossils occur,” Professor Flannery said.
“However, our studies of the tribosphenic molars of therian mammals found in Early and Middle Jurassic sediments from Madagascar, South America and India — which are up to 180 million years old — predate the oldest such remains from the northern hemisphere by 50 million years.”
“Furthermore, our new research shows that therian fossils from the Cretaceous of Australia, dating from around 126-110 million years ago, share characteristics with both these Jurassic southern hemisphere forms and the modern northern hemisphere therians.”
“The research has completely revised and turned on its head our understanding of early mammal evolution.”
During the Cretaceous period, Australia, New Zealand, South America, Antarctica, Africa, Madagascar, and the Indian subcontinent were all joined in one supercontinent, Gondwana.
Early mammal lineages must have existed at this time, but evidence of their presence in the fossil record, their anatomical features, and their evolutionary relationships have been slow to reveal themselves, preventing in-depth assessments until now.
“Our research indicates that Theria evolved in Gondwana, thriving and diversifying there for 50 million years before migrating to Asia during the Early Cretaceous epoch,” said Australian Museum’s Professor Kris Helgen.
“Once they arrived in Asia they diversified rapidly, filling many ecological niches.”
“A key component to the evolutionary success of Theria lies in their teeth. With their sophisticated molars, known as tropospheric molars, they were able to crush, puncture, and cut through food simultaneously.”
While the Eureka moment happened in comparisons made earlier this year, the research to find key mammal fossils has been painstaking.
The paleontologists searched Cretaceous-age rocks for over two decades before turning up the first mammal fossil.
“Dating mammal lineages depends on both genetic analysis and the fossil record. We also date the rock around the fossil,” said Museums Victoria’s Dr. Thomas Rich.
“With our latest research we have succeeded in filling the gaps to draw a detailed portrait of the early evolutionary history of Theria.”
“These astonishing series of discoveries have completely changed our long-held theory of mammal evolution. Indeed, it turns our ideas of mammal evolution on its head.”
The research is described in a paper in the journal Alcheringa.
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Timothy F. Flannery et al. The Gondwanan Origin of Tribosphenida (Mammalia). Alcheringa, published online November 1, 2022; doi: 10.1080/03115518.2022.2132288
Source link: https://www.sci.news/paleontology/gondwanan-origin-marsupial-placental-mammals-11490.html
