by Mary Caperton Morton Friday, December 21, 2018
A mother found fossilized alongside 38 of her young is offering a rare glimpse into early mammalian reproductive strategies. Unearthed in northeastern Arizona, the 184-million-year-old fossils are from specimens of Kayentatherium wellesi, an early mammal-like tritylodont that falls between reptiles and true mammals on the evolutionary tree.
A clutch of 38 offspring is a lot for a mammal, but reptiles tend to be more prolific reproducers. Additionally, the skulls of the young K. wellesi are similar in shape to that of the mother, a growth pattern more common to reptiles than mammals, whose skulls tend to lengthen as they mature.
“These babies are from a really important point in the evolutionary tree,” said Eva Hoffman of the University of Texas at Austin (UT), who co-authored a new study in Nature about the fossils with UT colleague Timothy Rowe, in a statement. In addition to their similarities to reptiles, she said, “they had a lot of features similar to modern mammals, features that are relevant in understanding mammalian evolution.”
The new specimens show that tritylodontids retained a primitive pattern of reproduction despite sharing various skeletal features with mammals, the team wrote. The association of large litter sizes with uniform cranial growth supports the notion that the evolution of bigger brains may have been the driving force behind later changes in mammalian reproduction and development, as mammals traded the advantages of larger clutch sizes for bigger brains.
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