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| Credit: Eva Hoffman / The University of Texas at Austin. |
More transitional fossils again this week, creationists. That 'non-existent' pile of 'non-existent' evidence just keeps getting bigger and bigger!
This time it's the discovery of a unique set of a female and her newborn young of a species of very early proto-mammal which still had some of the characteristics of their reptilian ancestor, one of which was the size of the litter. The adult female was found with 38 babies - more than double the average litter size of any living mammal. This is the only known example of fossils of the babies of a mammal precursor.
The discovery was made in Arizona about 18 years ago by Professor Timothy Rowe of the University of Texas Jackson School of Geosciences, but he didn't realised then what exactly he had discovered, thinking it was a single specimen of the beagle-sized long-extinct herbivore, Kayentatherium wellesi that coexisted with dinosaurs and lived about 165 million years ago. Kayentatherium wellesi had several features in common with modern mammals and probably had hair.
Then in 2009, Sebastian Egberts, a fossil preparator at the Jackson School spotted a tiny molar-like tooth that suggested there may be babies in the rock as well as an adult. But it wasn't until the advent of modern High-Resolution X-ray Computed Tomography, combined with image-processing technology that lead author, Eva Hoffman, a graduate student at Jackson School was able to examine the fossil-bearing rock to reveal just what Rowe had found.
In addition to the very large litter size, the skulls showed that Kayentatherium wellesi had a very small brain for a mammal and more typical of a reptile. Together with the large litter size, this suggests that part of the evolutionary trade-off for the relatively large mammalian brain was a reduction in litter size. Growing a large brain places a high energy burden on the female, whether the babies develop internally or in an egg. Within a few million years, the early mammals had larger brains and smaller litters.
Because of this large brain, mammal young typically have a bulbous head and a foreshortened (neotenous) face that develops into the adult face as the young grows (the human head and face is an exaggerated version of this neoteny which is retained into adulthood). Kayentatherium wellesi had no sign of this; the babies having a miniature version of the mother's skull.
It is not clear whether the babies were newly hatched or were near to hatching.
Hofmann's, and Rowe's findings are published today in Nature:
Abstract
Transformations in morphology, physiology and behaviour along the mammalian stem lineage were accompanied by profound modifications to reproduction and growth, including the emergence of a reproductive strategy characterized by high maternal investment in a small number of offspring1,2 and heterochronic changes in early cranial development associated with the enlargement of the brain3. Because direct fossil evidence of these transitions is lacking, the timing and sequence of these modifications are unknown. Here we present what is, to our knowledge, the first fossil record of pre- or near-hatching young of any non-mammalian synapsid. A large clutch of well-preserved perinates of the tritylodontid Kayentatherium wellesi (Cynodontia, Mammaliamorpha) was found with a presumed maternal skeleton in Early Jurassic sediments of the Kayenta Formation. The single clutch comprises at least 38 individuals, well outside the range of litter sizes documented in extant mammals. This discovery confirms that production of high numbers of offspring represents the ancestral condition for amniotes, and also constrains the timing of a reduction in clutch size along the mammalian stem. Although tiny, the perinates have an overall skull shape that is similar to that of adults, with no allometric lengthening of the face during ontogeny. The only positive allometries are associated with the bones that support the masticatory musculature. Kayentatherium diverged just before a hypothesized pulse of brain expansion that reorganized cranial architecture at the base of Mammaliaformes4,5,6. The association of a high number of offspring and largely isometric cranial growth in Kayentatherium is consistent with a scenario in which encephalization—and attendant shifts in metabolism and development7,8—drove later changes to mammalian reproduction.
Hoffman, E. A. & Rowe, T. B.
Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth.
Nature (2018). doi:10.1038/s41586-018-0441-3
Copyright © 2018 Springer Nature
Reprinted with kind permission under licence #4418800310084
So, these fossils show clear signs of transition from the reptile ancestors of these mammaliform reptiles to the mammals they would soon become. As good an example of a transitional species between two different orders of amniotes as one could wish for; and so another fossil for creationists to have to perform intellectual somersaults over so they can continue to believe the dogma rather than the evidence.
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