#enantiornithean

Adamantina Formation enantiornithean jaw from Wu et al 2021.

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)

A menagerie of materials associated with embryonic Gobipteryx and Gobipipus, Chatterjee et al 2013.

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

More material from Wu et al 2021

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

Yuornis material from Xu et al 2021.

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.

Falcatakely

From O’Connor et al 2020.

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

Yungavolucris and Halimornis by midiaou and xenopleurodon respectively. Both are real life Cretaceous taxa, showing that these birds were already diversifying into aquatic ecologies.

Enantiornithes are a group of extinct flying theropod dinosaurs that you could reasonably call birds, being the sister group of Euornithes (the group that includes modern birds). However, they differ from our birds in a variety of ways (their name literally means “opposite birds” for a reason):

Several skeletal details, including a tarsometatarsus that is either unfused or half-fused (beginning at the top rather than at the bottom, the opposite than in modern birds), an articulation of the scapula and coracoid that is oppositely shaped (hence the name; the coracoid joint is convex and the scapula joint is concave shaped in enantiornitheans, while the opposite happens in modern birds), a shallower sternum keel with bizarre antler-like projections (which, combined with large crests in their humerus, suggests the muscles lifting the wing were attached to the back as in bats and pterosaurs, rather than all flight muscles being attached to the keel as in modern birds), and a large, rod-shaped pygostyle (which will be relevant later).

Usually toothed jaws instead of beaks, though some taxa did become toothless. Even then, these weren’t capable of cranial kinesis like modern birds (i.e. watch a duck or your pet parrot yawn and you can see them moving their upper jaw; enantiornitheanss are many things but they’re not that abominatory).

All known taxa thus far seem to have been superprecocial: ample sites show buried eggs like those of megapodes, and the hatchlings were already fully flight capable soon after birth.

Unlike modern birds, enantiornitheans lacked a tail fan. They either had contour feathers on their butt like in the rest of the body or had long, streamer-like display feathers, also found in other Cretaceous bird groups but not in modern birds. Some species did have retrices, but they were arranged along the rod-like pygostyle and were not a movable fan, so essentially they were a variation of the tail fronds seen in Archaeopteryx and kin. Note that this did not make flight harder; even modern birds can fly reasonably well without a tail.

Why the opposite birds died out at the end of the Mesozoic while ours survived is unclear. Often, a bias towards arboreal niches is cited, as many enantiornitheans were in fact arboreal, but as the examples above show they also occured in marine and terrestrial niches alongside the ancestors of modern birds. Another possibility is their supreprecocial habits, meaning a more complex ecology as the birds matured since they were already functionally independent since birth, and this did hinder reptiles like lizards so the answer might lay there.

Or, most likely, it was just dumb luck.

Anyways:

Senmuruy hvare by Dave García. A four meter wingspan predator vaguely analogous to the golden eagle and cinnereous vulture, soaring across the northern hemisphere for corpses to dig its long snout into or live mammals and birds to sink its talons into.

Many Cretaceous enantiornitheans were already suspected of being raptorial, so it is only natural that, once pterosaurs were gone, they’d increase in size. Some reach wingspans of fiver meters, but most are more moderately sized at 1.5-3 meter adult wingspans. Smaller sizes are handled by the young, which like all enantiornithes can already fly since birth and occupy distinct ecological niches. Most species protect the nest and moderate its temperature like our megapodes, and a few even display mild parental care, allowing the young to remain in the vicinity until they’re large enough to be competition.

Euodontopteryx anatosuchus, a six-meter wingspan pelagic soarer that occurs in tropical and temperate waters, using its massive wings to ride on thermals like frigatebirds while landing to feed like albatrosses. Males sport streamer-like display feathers. By Dave García.

As noted above, some Cretaceous enantiornitheans were already aquatic, so this trend continued. Some species became divers, mostly wing propelled and some even flightless like our penguins, while others inversely invested in supreme gliding abilities, able to either ride thermals like frigatebirds or wave winds like albatrosses.

The most impressive species are reccord beaters. Divers can be as tall as a man when on land, while soarers can reach wingspans of over 7 meters, competing with flying multituberculates for largest living flying animals. Both groups tend to have long, toothy maws, the teeth alloted into a single row rather than individual sockets; this condition is known in both extinct sea birds and reptiles as well as some living cetaceans, and is known as aulacodonty.

Ghaltavis rex, a three meter tall predator that stalks African and Asian savannas. An apex predator of its own right, an echo of the distant unrelated tyrannosaurs in the form of a bird. By Dave García.

At least one real life enantiornithean, Elsornis, appears to have been flightless. It’s descendents were quick to occupy roles previously taken by non-avian theropods, from ratite-like herbivores to formidable predators that look like the fusion of a terror bird and a tyrannosaur, using their powerful jaws to crush bone.

The relatively long enantiornithean pygostyle allowed them to balance their pelvis/femur joints (a known size inhibittor in our birds) and grow to sizes larger than our timeline’s birds, though species above a ton are fairly rare seeing as mammals got their footing as well.

Bennu seti, a filter-feeding bird from Africa, Eurasia and Australia. Like flamingos it metabolizes carotenoids, giving it an orange colouration. By Dave García.

The Cretaceous Lectavis had long legs in some aspects convergent with those of flamingos. Thus, several enantiornitheans developed wading ecologies, ironically more associated with their euornithean competitors. Some became probers, dipping their maws (or toothless beaks) into the subtrate, while others became piscivores like herons or aquatic plant specialists like some cranes and magpie geese.

Most spectacular is a filter-feeding clade, Bennuidae. These birds modified their teeth into thin, delicate strands like some Cretaceous pterosaurs, and feed by swallowing water and expelling it, trapping prey in the teeth and keratinous spikes in the tongue. Having the nostrils still at the end of the snout, these birds usually feed in a different position from flamingos: rather than upside down, the lower jaw is submerged, in a manner similar to avocets.

Like most opposite birds the young are superprecocial, starting as plover-like birds before transitioning into a filter feeding lifestyle months later. Though some taxa form protective creches like flamingos, though unlike them they do not feed the young.

Bird feathers preserved in amber

The presence of "flight feathers" suggests the bird could fly when hatched.

It’s good to see a well-written, methodologically sound, open access paper (with supplementary material, excellent images, and quantifiable measurements) about an honestly impressive and spectacular discovery that tells us a lot about both soft tissue preservation and bird origins. And to have it recognized in the media-at-large and tumblr-at-large while encouraging additional discussion is just the icing on the cake, really.

I've now memorized how to spell and pronounce enantiornithean.