Acristatherium is not a eutherian. Maybe not even a therian.

Hu et al. 2016
recently published on Acristatherium yanensis (IVPP V15004, Fig. 1), which they described as a new basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China. The specimen includes most of the skull. They noted the presence of an unexpected septomaxilla (other eutherians do not have that bone). They tested 70 taxa and 408 characters. They nested “A. yanensis as the most basal eutherian in the selected group. The morphological differences between Acristatherium and Eomaia indicate that eutherians already had a significant degree of generic diversification ca 125 Ma (mya).”

Figure 1. Acristatherium nests with Cronopio, between monotremes and metatherians in the LRT, not with eutherians.

Figure 1. Acristatherium nests with Cronopio, between monotremes and metatherians in the LRT, not with eutherians.

Unfortunately they did not include
Cronopio, Monodelphis and Didelphis among their 70 taxa. In the large reptile tree Acristatherium nested with Cronopio and together they nested between monotremes and metatherians. The septomaxilla is commonplace around that clade. In both the original and LRT cladograms, Acristatherium nested just primitive to Eomaia. However, Hu et al nested the basal carnivoran eutherian, Vincelestes more primitively than Acristatherium + Cronopio for reasons unknown.

Figure 1. Cronopio is a basal mammal nesting between Juramaia and Didelphis, a basal marsupial.

Figure 2. Cronopio is a basal mammal nesting between Juramaia and Didelphis, a basal marsupial. Scale bar = 1cm.

Cronopio dentiacutus (Rougier et al. 2011, early Late Cretaceous, 98 mya, MPCA PV 454) was originally described as a dryolestid mammal. Tradtionally dryolestids nest outside of the Theria. That is also so in the large reptile tree as Cronopio nests between Juramaia and Didelphis. Here we see yet another diastema, a trait found in many diverse and often unrelated mammals. Acristatherium has no diastema and so was likely more primitive. The two nest as late survivors of an earlier Late Triassic radiation and were probably both commonplace in their time.

Therians do not lay eggs,
but give birth to live, often underdeveloped young. At present, it is not possible to determine via phylogenetic bracketing if Acristatherium laid eggs or produced underdeveloped young, but I would lean toward the latter given its tiny size.

PostScript:
Dr. Darren Naish recently praised online for SciAm the 1854 prehistoric Crystal Palace restorations by BW Hawkins (Fig. 3) as, and I quote, “Among the Most Accurate Renditions of Prehistoric Life Ever Made….Often derided, and today somewhat neglected and forlorn, the famous prehistoric animal models of Crystal Palace in London have a lot to teach us…”

Figure 3. One of the Victorian London dinosaurs crafted by BW Hawkins for the Crystal Palace exhibit, that Dr. Naish praises.

Figure 3. One of the Victorian London dinosaurs crafted by BW Hawkins for the Crystal Palace exhibit, that Dr. Naish reports, “have a lot to teach us.”

You might remember Dr. Naish was also the harshest and most vocal critic of the blog you are reading and the website it promotes, ReptileEvolution.com. He and others blackwashed the entire project as something “the world has to ignore,” despite every attempt at accuracy and transparency. Several years later, Dr. Naish is still wondering “When are you going to stop?”

Let us all remember, it is human nature to embrace and extol the overlooked nuances we believe we are the first to appreciate and talk about. That’s why scientists take such delight in the process of discovery, no matter how little it interests the rest of society. That’s why whenever we parents congregate we laud our own children. And that’s why 1995 hipsters embraced the Hush Puppies shoe just as the brand was hitting the nadir of its popularity.

Likewise, because we all have egos, it is also human nature to denigrate and disparage overlooked nuances that others were the first to appreciate and talk about, especially when those others do not have ‘hipster’ status and are metaphorically ‘swimming in the same pond’. That last factor is very important. That’s why alternate views of Christianity were exterminated for 15 centuries — even though they all followed the ‘Prince of Peace’. That is also why American females were not allowed to vote, become Mercury astronauts or run for president until alpha male attitudes relaxed — even though we males still loved American females as wives, daughters and mothers.

Perceived menace makes people kinder to their kin (race, gender, income level, religion, nationality, baseball team), but nastier to outsiders (see SciAm online article here). That’s why it is always better to convince others you share common values and origins. A century ago the Irish in America went through their own ‘freshman hazing’ and came out with an annual holiday. Gaining acceptance in the scientific community for the large reptile tree should not be that difficult since it likewise offers no threat, but helps test away many enigmas and misfits.

For more on primate status behaviors, according to Jane Goodall, click here.

References
Hu Y-M, Meng J, Li C-K and Wang YQ 2016. New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China. Proceedings of the Royal Society B.
doi:10.1098/rspb.2009.0203 Published online
Rougier GW, Apesteguía S and Gaetano LC 2011. Highly specialized mammalian skulls from the Late Cretaceous of South America. Nature. 479: 98–102. doi:10.1038/nature10591.

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5 thoughts on “Acristatherium is not a eutherian. Maybe not even a therian.

  1. “The Crystal Palace models are super-accurate. For all those statements about the models being laughably anachronistic, a strong argument can be made that they’re among the most accurate reconstructions of extinct animals ever produced. How can I say this? Because they really do reflect, to remarkably detailed degree, what leading scientists of the time thought about the life appearance of the animals concerned. Furthermore, they were designed and made by a person with a world-class knowledge of animal anatomy and appearance. I’m talking of Benjamin Waterhouse Hawkins.”

    If you present quotes in full they don’t look so ridiculous.

  2. Exactly, David! Some day, your work will be appreciated for what it is, IMO; a valid and valiant effort to add to our knowledge of prehistoric life. Keep up the good work!

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