Compendium of Enantiornithean Late Cretaceous Jaws

I still remember the Mirarce paper and its authors’ decisions to depict the eponymous bird with a toothless beak. This decision was based on an assumption that, by the Late Cretaceous, most Enantiornithes had lost their teeth, concurrent with increased speciations towards powered flight and large sizes.

Yet, in recent years it has become increasingly clear that there was no “end goal” for the evolution of toothlessness in birds, with many taxa remaining toothed until the very end of the Mesozoic. Thus, no specific reason for the opposite birds to have lost their teeth.

Still, fact of the matter is that very few cranial remains identified as Enantiornithes date to the Late Cretaceous. While most sepcimens are predictably fragmentary, a few are almost complete, rendering this a frustrating puzzle. For example. Neuquenornis retains a fairly complete skeleton aside from the jaws, a skull well preserved aside from the front. Classic spiteful gods.

Hence, I’ve decided to make a small compendium of known enantiornithean jaw material from the Late Cretaceous.

Gobipteryx (and Gobipipus)

For most of history the most well preserved Late Cretaceous enantiornithean material came from Asia. The taxa Gobipteryx occurs in Campanian-aged deposits of the Gobi Desert, and includes a myriad of exquisitely preserved material ranging from adults to embryonic remains. A lot of our understanding of the lifecycle of Enantiornithes in fact comes from these animals, hatchlings being supreprecocial and able to fly nigh immediately after birth.

Another more controversial taxon is Gobipipus, known from much the same deposits. Its known almost exclusively from embryonic specimens and several researchers have argued that it differs substantially from embryos assigned to Gobipteryx, but this debate is on-going.

Both birds lack teeth, instead having a keratinous beak whose upper jaw curves upwards. The bony components of the beak differ drastically from those of modern birds, with the maxilla being well developed and forming a large part of the upper jaw margin instead of being reduced as in modern birds, and it’s still unclear if it was capable of cranial kinesis like modern birds do. The ecology of these animals is also rather unclear; they come from what were in life arid environments, but some have suggested a piscivorous lifestyle for these birds, which would be in line wth some studies finding them closely related to the piscivorous longipterygids and Halimornis (see supplementary material). Maybe some sort of seagull-like ecology, foraging in desert lakes?

Regardless, this painted a picture for Late Cretaceous Enantiornithes, and no doubt inspired the decision of the peeps on the Mirarce paper. Thankfully, other, more recent discoveries seem to be putting this to rest.

Adamantina Enantiornitheans

The Adamantina Formation dates to the Late Cretaceous, somewhere between the Campanian and the Maastrichtian depending on estimates. A partcular quarry, known as “William’s Quarry”, has wielded a massive amount of fairly well preserved enantiornithean fossils. These birds have not yet been described, but they are so complete that a study about their tooth replacement patterns was even possible.

Unlike the Mongolian birds, these ones clearly have teeth. Curiously, their snout shape is rather similar to that of modern raptors, the jaws ending on a hook. However, unlike contemporary birds like Ichthyornis, these hooks end not in a beak, but still host teeth, which is frankly amazing. I’m assuming they probably were hawk or falcon like animals, but given their rather unique snout morphology a more specialised diet like that of snail kites is also a possibility.

These animals clearly prove that toothed opposite-birds endured until the end of the Mesozoic, and considering avisaurids have typically been reconstructed as raptor-like birds I’m assuming Mirarce probably also had teeth.

Yuornis

In 2021 a brand new completed enantiornithean was found in Henan, China. Roughly contemporary to the Mongolian birds, the ensuing phylogenetic study actually groups Yuornis with them, but the authors rejected this as bias due to toothlessness and elected to not make it part of Gobipterygidae.

Like gobipterygids Yuornis lacks teeth, but has a substantially different beak morphology, hence why the reluctance to consider it closely related t them. For starters its maxilla is more reduced (albeit much larger than in modern birds) while its premaxilla approaches the modern condition. Its beak is also rather narrow, and does not curve upwards. Combined with strong wings and perching feet, this seems like a Mesozoic analogue for a small corvid like a jay or magpie. No mentions of cranial kinesis are made, but several palatal elements are similar to more derived birds so it might have been able to do so.

If unrelated to gobipterygids, Yuornis represents a second lineage of toothless opposite-birds. This is not unsual as birds as a whole lost teeth multiple times and the same likely applied to Enantiornithes, but its clear by now that this was not the norm for the last enantiornitheans.

Facatakely

Falcatakely was found just two years ago and shows one of the most derived Mesozoic avian beaks of all time, with a maxive maxila and nasal while the premaxila is tiny, the polar opposite of modern birds. Small peg-like teeth line the end of the jaws, while the rostrum itself is deep and curved, resembling that of a toucan. It would then join hornbills and true toucans in the convergent evolution club.

But there’s a reason I left this for last. Several reseachers are not convinced it is actually an enantiornithe, with a viable alternative by Mickey Mortimer being an omnivoropterygid. Sapeornis like birds are known from Falcatakely‘s Maastrichtian locale in Madagascar, but so are pengornithid enantiornitheans (which coincidentaly match O’Connor et al 2020’s phylogenetic results for this bird). In the end, more evidence will be needed to determine it either way.

Conclusion

Its clear that Late Cretaceous Enantiornithes had a wide variety of lifestyles and ecologies, and with that came a variety of jaw anatomies. Some groups did indeed become toothless, but it is patently clear that many toothed species lived all the way to the end.

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