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circlesfan:

People who know will definitely tell you how much I’ve come to
appreciate Mesozoic mammals. Usually dismissed as small rat things,
mammals in the Mesozoic were a highly diverse bunch of animals,
including swimmers, diggers, anteater like forms, large terrestrial
predators, hoppers and many, many more.

Of these, eutriconodonts are by far among the more spectacular. I’ve already talked at length about the possible flight capacities of volaticotheres, but really the whole clade is pretty neat, and here’s why:

1- The first mammalian carnivores

Jugulator amplissimus by @paleoart

Eutriconodonts are notable for being among the first mammals specialized to dedicated carnivory. Zofia Kielan-Jaworowska
identified numerous features associated with obligate carnivory: long,
sharp canines (or canine-like incisors in the case of gobiconodontids),
premolars with trenchant main cusps that were well suited to grasp and
pierce prey, strong development of the mandibular abductor musculature,
bone crushing ability in at least some species and several other
features.

Their iconic triconodont dentition, usually taken as “primitive”,
might actually be specialized for shearing (Zofia Kielan-Jaworowska
2004, Sigogneau-Russell 2016), making it vaguely analogous to the
carnassials of placentals and marsupial predators. Their exact shearing
mechanism has no real analogue among mammalian carnivores, but the
function is considered very similar at least (Rougier 2015)

Equally important is eutriconodont size. Eutriconodonts are among the
largest mammals in Mesozoic faunal communities, which has been inferred
as standing the highest among mammals in contemporary trophic webs (
Zofia Kielan-Jaworowska 2004). At their size, they were perfectly
capable of taking down vertebrate prey, and the largest gobiconodontids
like the infamous Repenomamus might have been apex predators in their environment.

Repenomamus itself has been found with dinosaur remains in its belly, and scavenging marks associated with Gobiconodon
have also been found. These mammals could, in fact, tackle dinosaurs,
and if modern analogues like wolverines, tasmanian devils and ratels are
of any indication then the largest eutriconodonts could in fact be “top
guns” in their environments.

Other Mesozoic synapsids have also been inferred to be specialized carnivores, like Sinoconodon and deltatheroideans.
But they lived either before eutriconodonts spread, or after they
became extinct, and as such their range was much more limited.

2- Their diversity

Speculative depictions of Ichthyoconodon by @alphynix. While I argue for a slightly different lifestyle, they help summarize the range of known eutriconodont bauplans, such as the otter like Yanoconodon and Liaoconodon and the aerial Volaticotherium and Argentoconodon

Better only than carnivorous Mesozoic mammals are carnivorous
Mesozoic mammals that come in all shapes and sizes. In spite of being
pretty much highly specialized carnivores and certainly more restricted
in terms of diet than, say, symmetrodonts or early therians,
eutriconodonts were much more diverse than these groups were (until
therians got them beat after eutriconodonts went extinct, that is).

The group ranged from shrew analogues (amphilestids, amphiodontids, basal gobiconodonts and some triconodontids), arboreal, tree-shrew like forms (Jeholodens), large, robust carnivores (gobiconodontids, Jugulator, triconodontids), a quilled species with an immensely thick spine (Spinolestes), at least two lineages of swimmers (Liaoconodon and Yanoconodon) and of course the aforementioned volaticotheres, conservatively gliders if not outright flyers.

The exact smallest triconodonts probably weighted around 50 grams. The largest, Repenomamus giganticus, as much as 14 kg.

This level of ecological diversity is so far unmatched by any
Mesozoic mammal group save for multituberculates and perhaps Late
Cretaceous metatherians. It is even larger than the diversity of most therian carnivore groups, save for carnivorans.

Every possible niche taken by carnivorous mammals under 14 kg was taken, and it’s amazing.

3- Brains!

Yes, we know about eutriconodont brains. In fact, Triconodon mordax is one of the first extinct animals to have its endocast studied (Simpson 1928).

From what we can tell, at least from this one specimen,
eutriconodonts had fairly “primitive” brains for mammal standards. The
cerebral hemisphere is long, oval and flat, lacking the inflated
appearance present in modern mammals (including monotremes, which are
generally held to be more basal than eutriconodonts!) as well as the
also extinct multituberculates. The cerebrum is similarly not expanded as much as in those groups. Like multies, Triconodon has a large, semi-triangular bulge, thought to be a large cistern.

What this means about eutriconodont intelligence is unclear. It might
seem like they were fairly stupid mammals, but mammals with fairly
simplistic brains are known to be fairly intelligent (Weisbecker 2010).
They probably weren’t as cunning as modern cats and dogs, but probably
capable of complex behaviors nonetheless.

4- Everywhere For A Long Time

Different mammaliaform tooth types across the Mesozoic. Eutriconodonts, alongside the unrelated morganuconodonts, were the only mammals to bear a “triconodont” tooth type.

While eutriconodonts fall short of multituberculates as the longest
living mammal lineage, they were still very successful. The first
eutriconodont fossils – Argentoconodon, Victoriaconodon and Huasteconodon – all date to the Toarcian and represent a large variety of lineages, indicating an even earlier origin.

Eutriconodonts would then keep on going in full force for another 111
million years. Even when other mammal groups display gaps in their
fossil record, eutriconodonts continue across fossil sites in Europe,
Asia, North America, Africa and South America, rendering them a truly
global presence in Mesozoic faunas as much as dinosaurs and pterosaurs.

Alas, they faced a final challenge with the spread of angiosperm
plants, which drastically altered faunal components across the globe and
was particular harsh on carnivorous mammals. Only one lineage survived
the Turonian, Alticonodon, to still endured all the way to the Campanian.

5- Poison-Heels

Comodon by @paleoart. Notice the spurs on the heels.

Okay, not something exclusive to eutriconodonts among mammals, but it bears repeating.

Venomosity is inferred to be an ancestral trait for mammals (Hurum
2006). Various Mesozoic mammal groups possess heel spurs similar to
those of the modern platypus, which delivers a powerful neurotoxin
infamous for how painful it is. This includes similar canals, which
implies an identical function.

This spurs have been found in nearly all non-therian mammal groups,
suggesting that either venom evolved multiple times among Mesozoic
mammals, or, most likely, that it was an ancestral feature later lost in
therians.

Eutriconodonts, of course, preserve such spurs. They are best known
in gobiconodontids, which combined with other features would make these
some of the most ridiculously over-engineered killing machines of the
time.

6- Tough As Nails

Fantastic Mr. Spinolestes.

Gobiconodontids were probably the honeybadgers of the Jurassic (and
early Cretaceous). The largest of all eutriconodonts, they included not
only the infamous Repenomamus, but the also fairly sizeable Gobiconodon. These animals are racoon-to-wolverine sized beasts, bearing thick skeletons, robust jaws and sharp fang-like incisors.

As such, not only were they large carnivorous mammals for the time,
but also specifically designed to fight violently. Combined for evidence
for scavenging for Gobiconodon and outright dinosaur-consumption for Repenomamus,
it’s hard to not see these as competitors for small to mid-sized
theropod dinosaurs in their local environments. “Small”, but incredibly
brute fighters, fighting their way into carcasses and perhaps even
harassing fellow predators.

That said, even the smaller gobiconodontids were nothing to laugh at.

Spinolestes, a more conventionally shrew-sized animal, bears:

– A massively thick, xenarthrous spine similar to that of xenarthrans and the hero shrew. This probably allowed it to survive being smashed by animals up to 75 kg.

– Spines similar to those of the modern spiny mice.

– The venom spurs.

What can you even say to that?

Conclusion

I think Mesozoic mammals are underrated in general, but
eutriconodonts in particular are a very fascinating group. Besides these
undeniably awesome facts, there’s also the fact that they bear some of
the most exquisitely preserved Mesozoic mammal fossils, something even
the more well known multituberculates currently lack.

Dismissed as just archaic “missing links”, they were a dynamic,
fascinating group of animals, which I believe deserve some recognition.

References

Zofia Kielan-Jaworowska, Richard L. Cifelli, Zhe-Xi Luo (2004). “Chapter 7: Eutriconodontans”. Mammals from the Age of Dinosaurs: origins, evolution, and structure. New York: Columbia University Press. pp. 216–248. ISBN 0-231-11918-6.

Percy M. Butler; Denise Sigogneau-Russell (2016). “Diversity of
triconodonts in the Middle Jurassic of Great Britain” (PDF).
Palaeontologia Polonica 67: 35–65. doi:10.4202/pp.2016.67_035.

Chen, Meng; Wilson, Gregory P. (2015). “A multivariate approach to infer locomotor modes in Mesozoic mammals”. Paleobiology. 41 (02): 280–312. doi:10.1017/pab.2014.14. ISSN 0094-8373.

Vera Weisbecker and  Anjali Goswami, Brain size, life history, and metabolism at the marsupial/placental dichotomy, Proc Natl Acad Sci U S A. 2010 Sep 14; 107(37): 16216–16221. Published online 2010 Sep 7.   doi:  10.1073/pnas.0906486107

For a long time, our understanding of mammals during the “Age of Reptiles” was that they were rat-like and lived in the shadows of giants waiting for their chance to rise up. Nowadays we have a more complete appreciation for these early mammals!