Ichthyoconodon

Ichthyoconodon jaworowskorum holotype.

Its been well over an year in which I have discussed extensively the possibility of flying volaticotheres, both here as well as on other sites. The catalyst for this argument — and long brought up in The Speculative Dinosaur Project’s yahoo group — is the pair of molar teeth that compose the holotype of Ichthyoconodon jaworowskorum.

These teeth, dating to the Berriasian, occur in a marine, littoral environment that now is the Anoual Syncline beds of Morocco. In and of itself, this shouldn’t be too odd: mammal fossils, even small mammals, do occur in marine deposits, dragged down by rivers. These same beds also bear fossils of mammals such as amphilestids, Gobiconodon, various dryolestoids and even a group of non-mammalian cynodonts, the hahnodontid haramiyidans, all alongside shark, sea turtle and ray-finned fish fossils as well as other animals aquatic and terrestrial, from lizards to theropods.

However, Ichthyoconodon bears the distinction of having its molars not showing any signs of water-derived erosion. This is in stark contrast to contemporary mammal fossils, which in fact do, aside from the contemporary Dyskritodon (Sigogneau-Russell 1995). For this reason, Ichthyoconodon must have died either in situ or dragged only a short distance from it’s death place.

Sigogneau-Russell interpreted this as an indication that the animal was aquatic, and even compared the teeth to those of seals and cetaceans. Zofia Kielan-Jaworowska, however, has later posited that such an equivalency might be misguided, as eutriconodont molars served as slicing, carnassial-like tools rather than the piercing and grasping teeth of marine mammals. Nonetheless, more recently the eutriconodont Liaoconodon has been proven to be an aquatic animal (Men et all 2015), so maybe this supposed lack of equivalency might be a red herring. Otters and sea lions do still have slicing molars, after all.

Then, the discovery of Volaticotherium antiquum in China in 2006 and a further examination of the Argentinian Argentoconodon fariasorum in 2011 brought a new possibility. The molars of both of these animals were extremely similar to those of Ichthyoconodon, albeit with more curved cusps, clearly indicating that it was closely related to these animals, having first been understood as the sister taxa to Volaticotherium (Meng 2006) and then as the immediate outgroup to the Volaticotherium — Argentoconodon clade (Gaetano 2011). While the less curved cusps and larger size clearly imply some degree of speciation in relation to these animals, overall it seems likely that Ichthyoconodon was similar to Volaticotherium and Argentoconodon.

The thing is, these animals — originally Volaticotheria, then degraded to Volaticotherinae/Volaticotherini depending on how they’re placed within Triconodontidae; recent studies favor them in a more basal position within Eutriconodonta (Averianov 2011, Martin 2015), so Volaticotheridae or Volaticotheria are both valid, and so I use volaticothere — are not aquatic, but rather specialized to an aerial lifestyle. Volaticotherium preserves an extensive patagium, a specialized femur that constraints terrestrial movement but can endure flight stresses and canines similar to those of predatory bats (Meng 2006), with Argentoconodon sharing most of these traits (aside from the patagia, sadly not preserved).

I’ve already explained in detail why flying volaticotheres make sense, but Ichthyoconodon‘s conundrum has to be the most unique and perhaps final piece of evidence. It is a fossil of an animal that died at sea and unique in having such well preserved teeth. It had to have been carried over a short distance at most.

Most likely, this was a flying animal, that died at sea.

Of course, in the absence of more complete volaticothere materials, this is a pretty extreme claim. Some people have cautiously proposed other explanations, with @synapsid-taxonomy on Tumblr having produced a fairly neat compilation:

”

• Ichthyoconodon was an aquatic mammal, died at sea, sunk and was buried.
• Ichthyoconodon was a terrestrial/arboreal/gliding mammal that died on the shore, was washed out for a short distance, sunk and was buried.
• Ichthyoconodon was a gliding mammal that was caught in a storm and blown out to sea, sunk and was buried.
• Ichthyoconodon was a terrestrial/arboreal/gliding mammal that got stuck on a floating raft of vegetation, fell into the sea, sunk and was buried.
• Ichthyoconodon was a mammal that was grabbed as prey by a pterosaur and dropped over the ocean, sunk and was buried.

”

Without further ado, let’s investigate these claims and why I don’t think they hold ground.

Aquatic Mammal Hypothesis

Liaoconodon hui by Dylan Bajda.

The original hypothesis by Sigogneau-Russell, this one has been criticized by Zofia Kielan-Jaworowska as seen above, but with the advent of Liaoconodon I think it might hold some weight after all.

Ichthyoconodon, while a volaticothere by definition in both Meng 2006 and Gaetano 2011 (“last common ancestor between Ichthyoconodon and Volaticotherium“), is still outside the Argentoconodon + Volaticotherium clade, the one with confirmed flight adaptations. This leaves a massive ghost lineage (it dates to the Early Cretaceous while its closest relatives date to the Early and Mid-Jurassic respectively, and their last common ancestor must have existed further back in time) that could have resulted in entirely different ecological speciation, as well as the very possibility that it never developed flight in the first place, something that could be restricted to the Argentoconodon + Volaticotherium clade.

Thus, the idea that Ichthyoconodon was an aquatic mammal cannot be ruled out. Personally, however, I think it is uniquely based on two criteria:

• Rarity: so far, only two Ichthyoconodon teeth have been found. If it was an aquatic animal, chances are more specimens — including must more complete material — would have been preserved. A single well preserved specimen is consistent with terrestrial flying animals that died out at sea, like some azhdarchid remains (Mark Witton 2008).
• Ambiquous speciation in relation to its relatives: Ichthyoconodon molars are still rather similar to those of Argentoconodon and Volaticotherium. They do differ in that they have less recurved cusps, and the animal appears to have been slightly larger (Meng 2006, Gaetano 2011). We do know that Volaticotherium and Argentoconodon occupied rather different carnivorous or insectivorous niches (David 2013), and Ichthyoconodon likely did too. However, the overall dental similarities seem to imply a similar ecological morphospace, and I find it unlikely that a piscivorous, non-volant Ichthyoconodon wouldn’t bear more specialized teeth.

Dragged by Waves

Poor pupper (artist unknown).

By far I consider this the least likely possibility due to two key factors:

• First, if the animal was dragged by the waves, we would have seen marks of erosion on the teeth, which explicitly are not there.
• Second, one would expect the corpse to be deposited further up the beach instead of in deeper waters, especially given its proportionally smaller size. We have evidence of this happening with other local animals, including maniraptor dinosaurs (Gauthier 1986).

Case closed.

Glider Caught In A Storm

Mother sugar glider calculating launching position.

Also rather unlikely given how modern gliding mammals work. Gliding mammals universally select their landing locations prior to launching (Jackson 2012); being unable to, you know, fly, they are fairly limited in regards to their locomotion, and rarely if ever change landing locations once on air. Gliding, after all, evolved as a means of safe jumping, so it makes sense that most gliding mammals don’t take risks.

To know knowledge, I have never come across reports of gliding mammals being blown away by storms. Surely the presence of gliders in insular environments would be much greater than it is now if these mammals were so easily caught by winds. But instead, gliding mammals simply refuse to glide in turbulent weather and seek shelter (Jackson 2012), making it unlikely that they are caught in storms.

The utter absence of fossils of purported gliding mammals in marine settings pretty much does it for me.

Rafting

(Artist unknown)

Now this makes much more sense. Animals, after all, do frequently get stuck on rafts of vegetation, something that has allowed countless trans-oceanic travels and invasions. Maybe an Ichthyoconodon simply landed on such a raft and died, either of starvation or thirst or by falling into the water and drowning.

My issues is that I don’t think we have proper examples of raft “crew” fossils. Certainly, with the abundance of rafts — especially in the Mesozoic, when giant crinoids vastly expanded surface ecosystems — one would expect that fossils of terrestrial animals in deeper waters would be more common. Have have the aftermaths of such trips, like monkeys in America and Compsognathus in Solhofen, but no fossil monkeys and dinosaurs that died in situ at sea.

To my knowledge, Ichthyoconodon would be the only fossil mammal example of a raft dweller, which decreases significantly the possibility of it being one.

Eaten By A Pterosaur

Ichthyoconodon jaworowskorum restoration by Dylan Bajda.

This is definitely a very interesting hypothesis. Pterosaurs, unlike modern raptors which tend to grab prey by the talons, caught their victims with their jaws, much like many other predatory birds such as frogmouths and storks as well as nearly all other carnivorous sauropsid.

In general, this meant that most pterosaurs swallowed their prey whole and immediately. There were exceptions — eudimorphodontines were unique in having developed chewing, while macropredators like Thalassodromeus probably tore their victims to shreds (Mark Witton 2013) — neither likely culprits given the spatio-temporal distance. Pterosaur ingestion would be pretty much out of the way, given the degradation the stomachal acids would give the teeth, unless pterosaurs were capable of producing pellets like modern birds and bats.

In short the only way this scenario to would would be that the pterosaur only had the mammal on its jaws for a brief moment of time, or that it ingested its victim and produced a pellet. This scenario would have happened much more easily if Ichthyoconodon was volant, as this sort of behavior has been seen with coastal birds attacking bats.

Conclusion

Dyskitodon amazighi teeth. Another eutriconodont met in the same odd circumstances.

As you can see, one two of the proposed alternate explanations hold much ground:

• Its unclear just how divergent Ichthyoconodon is from other volaticotheres.
• Waves don’t work that way
• Gliding mammals do not get caught in storms
• There is no evidence of fossil raft animal communities
• Pterosaurs could have only preserved the teeth if they could make pellets.

So, for now, I think the flying Ichthyoconodon hypothesis holds more water. Combined with the traits for volancy in other volaticotheres I’ve discussed lately, I can’t wait for more complete fossils.

References

Sigogneau-Russell, Denise (1995). “Two possibly aquatic triconodont mammals from the Early Cretaceous of Morocco” (PDF). Acta Palaeontologica Polonica. 40 (2): 149–162.

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.

Meng Chen, Gregory Philip Wilson, A multivariate approach to infer locomotor modes in Mesozoic mammals, Article in Paleobiology 41(02) · February 2015 DOI: 10.1017/pab.2014.14

Meng, J.; Hu, Y.-M.; Wang, Y.-Q.; Wang, X.-L.; Li, C.-K. (2007). “Corrigendum: A Mesozoic gliding mammal from northeastern China”. Nature 446 (7131): 102. Bibcode: 2007Natur.446Q.102M. doi:10.1038/nature05639.

Gaetano, L.C.; Rougier, G.W. (2011). “New materials of Argentoconodon fariasorum (Mammaliaformes, Triconodontidae) from the Jurassic of Argentina and its bearing on triconodont phylogeny”. Journal of Vertebrate Paleontology. 31 (4): 829–843. doi:10.1080/02724634.2011.589877.

A. O. Averianov and A. V. Lopatin. 2011. Phylogeny of Triconodonts and Symmetrodonts and the Origin of Extant Mammals. Doklady Biological Sciences 436:32–35 [M. Uhen/M. Uhen]

Martin, Thomas; Marugán-Lobón, Jesús; Vullo, Romain; Martín-Abad, Hugo; Luo, Zhe-Xi; Buscalioni, Angela D. (2015). “A Cretaceous eutriconodont and integument evolution in early mammals”. Nature. 526: 380–384. PMID 26469049. doi:10.1038/nature14905.

Witton, Mark P.; Naish, Darren; McClain, Craig R. (28 May 2008). “A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology”. PLoS ONE. 3 (5): e2271. PMC 2386974

References

. PMID 18509539. doi:10.1371/journal.pone.0002271.

David M. Grossnickle, P. David Polly, Mammal disparity decreases during the Cretaceous angiosperm radiation, Published 2 October 2013. doi:10.1098/rspb.2013.2110

J. A. Gauthier. 1986. Saurischian monophyly and the origin of birds. The Origin of Birds and the Evolution of Flight, K. Padian (ed.), Memoirs of the California Academy of Sciences 8:1–55

Jackson, Stephen Matthew and Schouten, Peter. Gliding Mammals of the World, Csiro Publishing, 2012

Mark P. Witton (2013), Pterosaurs: Natural History, Evolution, Anatomy, Princeton University Press, ISBN 978–0–691–15061–1