The transition from pre-mammals to early mammals
occurred during the Triassic era, despite the fact that many extremely rare pre-mammals, are known from post-Triassic strata (Fig. 1), but, so far, not post-Cretaceous strata.
Figure 1. Pre-mammal synapsids in the LRT colorized chronologically. The extreme rarity of these often late-surviving fossils is indicated by the scattershot chronology of sister taxa.
The new tiny Origolestes (Mao et al. 2019; Fig. 2) is an Early Cretaceaous late survivor of a Triassic radiation of pre-mammals. Therefore it is NOT in the lineage of the Mammalia, but retains traits from that clade.
Figure 1. Origolestes in situ with colors added using DGS methods. Note the tiny canines and odd coronoid process. These are indicators of a parallel evolution.
From the Mao et al. abstract:
“Based on multiple 3D skeletal specimens we report a new Cretaceous stem therian mammal that displays decoupling of hearing and chewing apparatuses and functions.
Just three days ago we looked at a similar situation in Jeholbaatar, only in that case the loose ear bones represented a reversal. Similar morphologies MUST be seen in a phylogenetic context.
“The auditory bones, including the surangular, have no bone contact with the ossified Meckel’s cartilage; the latter is loosely lodged on the medial rear of the dentary.”
“This configuration probably represents the initial morphological stage of the definitive mammalian middle ear.
Except that it arrives way too late in the fossil record.
Evidence shows that hearing and chewing apparatuses have evolved in a modular fashion. Starting as an integrated complex in non-mammaliaform cynodonts, the two modules, regulated by similar developmental and genetic mechanisms, eventually decoupled during the evolution of mammals, allowing further improvement for more efficient hearing and mastication.”
This represents a parallel evolution of the decoupling of the middle ear bones. Let’s see how the popular press represented this finding, led by the authors.
From an online ScienceNews.org article by Carolyn Gramling:
“Exceptionally preserved skulls of a mammal that lived alongside the dinosaurs may be offering scientists a glimpse into the evolution of the middle ear.”
We don’t need another ‘glimpse’. We know exactly how this happened in the pre-Late Triassic ancestors of mammals.
“The separation of the three tiny middle ear bones — known popularly as the hammer, anvil and stirrup — from the jaw is a defining characteristic of mammals. The evolutionary shift of those tiny bones, which started out as joints in ancient reptilian jaws and ultimately split from the jaw completely, gave mammals greater sensitivity to sound, particularly at higher frequencies. But finding well-preserved skulls from ancient mammals that can help reveal the timing of this separation is a challenge.”
All true and good background for a popular article.
“Now, scientists have six specimens — four nearly complete skeletons and two fragmented specimens — of a newly described, shrew-sized critter dubbed Origolestes lii that lived about 123 million years ago. O. lii was part of the Jehol Biota, an ecosystem of ancient wetlands-dwellers that thrived between 133 million and 120 million years ago in what’s now northeastern China.”
“The skulls on the nearly complete skeletons were so well-preserved that they were able to be examined in 3-D, say paleontologist Fangyuan Mao of the Chinese Academy of Sciences in Beijing and colleagues. That analysis suggests that O. lii’s middle ear bones were fully separated from its jaw, the team reports online December 5 in Science.”
So they’re setting up a narrative without the proper background that mammals with this configuration first appeared in the Late Triassic, tens of millions of years earlier. Then they talk to another expert.
“This paper describes a spectacular fossil,” says vertebrate paleontologist Zhe-Xi Luo of the University of Chicago, who was not involved in the new study. But he’s not convinced that O. lii represents an evolutionary leap forward in mammalian ear evolution.
“Luo notes that O. lii is closely related to the mammal genus Maotherium, which lived around the same time and in roughly the same location. In Science in July, Luo and colleagues reported that a new analysis of Maotherium revealed that its middle ear bones were still connected to its jawbones by a strip of cartilage (SN: 7/18/19).
That finding, Luo says, was expected. Maotherium is well-known as a transitional organism, in which the middle ear bones had begun to rotate away from the jaw but were still loosely connected by that cartilage. There are numerous branches and twigs on the mammal family tree, Luo says, and evolution occurred at a different pace on them. But, he says, it’s unlikely that O. lii would have had separated ear bones when Maotherium didn’t, given the pair’s close positioning on the tree.”
Still no mention of the Late Triassic origin for mammals and the parallel development in this late survivor.
“Luo says he also doesn’t find the study’s evidence that the separation was complete in O. lii convincing. Three of the four skulls in the study were missing all or part of the middle ear, and the gap between the middle ear bones and jaw in the fourth skull may have been a break that occurred during fossilization, he adds.”
See how paleontologists try to put the brakes on the work of their colleagues?
“However, the new study’s researchers reject this idea. “It’s common that different interpretations may exist for a discovery in paleontology,” says vertebrate paleontologist Jin Meng of the American Museum of Natural History in New York, a coauthor of the study. But, Meng says, none of the ear bones or the cartilage in any of the skulls show fractured or broken edges. That, he says, suggests that these features were already separated in the animals before their demise.”
See how paleontologists try to bounce back from criticism? Meng is correct. Such ‘different interpretations’ are common. I have them. Others have them. You have them. In any case, the indisputable late appearances of Origolestes and Maotherium attest to their removal from the origin of the Mammalia. What they can offer us is a parallel look at this chapter in synapsid evolution. In other words, they are not the main attraction. They are a side show.
References
Mao F, Hu Y-M, Li C-K, Wang YQ, Chase MH, Smith AK and Meng J 2019. Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal. Science eaay9220 (advance online publication). online here
An ancient critter may shed light on when mammals’ middle ear evolved
wiki/Zhangheotheriidae
