Anteavis, new theropod from Carnian (Late Triassic) of Argentina

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Anteavis crurilongus gen. et sp. nov.

Ricardo N. Martínez, Carina E. Colombi, Martín D. Ezcurra, Diego O. Abelín, Ignacio Cerda & Oscar A. Alcober (2025)
A Carnian theropod with unexpectedly derived features during the first dinosaur radiation
Nature Ecology & Evolution (advance online publication)
doi: https://doi.org/10.1038/s41559-025-02868-4
https://www.nature.com/articles/s41559-025-02868-4



The early diversification of dinosaurs produced a major ecological change in the terrestrial ecosystems, culminating with tetrapod assemblages dominated in abundance by dinosaurs by the Triassic/Jurassic boundary (~201 million years ago (Ma)). Therefore, studying the initial diversification of dinosaurs is crucial to understand the establishment of Mesozoic assemblages. However, the lack of stratigraphically continuous fossil data in the few geological units that preserve the oldest known dinosaurs (~233–227 Ma, Carnian age) obscures our understanding of this initial diversification. The Ischigualasto Formation in northwestern Argentina (231.4–225.9 Ma) yields a rich vertebrate assemblage and new studies resulted in an abundant and stratigraphically near-continuous fossil record, which offers new insights into the early diversification of dinosaurs. Among the discoveries, we report Anteavis crurilongus gen. et sp. nov., an early-diverging theropod, which supports the notable diversity of small- to medium-sized dinosaurs during the late Carnian. Anteavis is recovered outside Neotheropoda, but it has features previously thought to be exclusive to that group. We show that dinosaur diversity and abundance in the Ischigualasto Formation were higher than previously recognized, particularly among small herbivores (<30 kg) and medium-sized (30–200 kg) predators. This diversification occurred in Ischigualasto during a climatic shift to semi-arid conditions, but the return of more humid conditions resulted in a gap in the dinosaur record that started at 228.91 ± 0.14 Ma. Only 15 million years (Myr) later, in the middle Norian age, the dinosaur record recovered its abundance and diversity in the basin, but now it was characterized by larger-bodied species. Our findings demonstrate an early dinosaur diversification probably punctuated by a climate-driven faunal turnover in, at least, southwestern Pangaea.

***

News:

Dawn of the Dinosaurs: A Discovery Forces Rethinking Their Early Evolution (in Spanish)
El amanecer de los dinosaurios: un hallazgo obliga a repensar su evolución temprana


https://www.agenciasinc.es/Reportajes/El-amanecer-de-los-dinosaurios-un-hallazgo-obliga-a-repensar-su-evolucion-temprana

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NOTE: Latin avis is feminine so the species should be crurilonga.

[Ben Creisler]

Now with free pdf link:

https://www.researchgate.net/publication/396498867_A_Carnian_theropod_with_unexpectedly_derived_features_during_the_first_dinosaur_radiation

[Gregory Paul]

It was bound to happen someday and somewhere, and it has! 

As you recall, many moons ago, in DoA, I coined Avepoda. Which is a character plus clade group in which the key attribute is loss of contact of the inner metatarsal with the tarsals, producing the beginnings of the tridactyl grade of Theropoda foot. This contains Neotheropoda, which is further along the cladogram as a node-based clade of the common ancestor of Coelophysis and the rest of the tridactyls. 

Neotheropoda has the obvious limitation that is automatically excludes the basal-most tridactyl theropods. Which includes a whole lot of Triassic footprints. And means that if one is discussing the evolution of the tridactyl pes, using neotheropod to do so is automatically excludes the tridactyls basal to neotheropods. 

Avepoda should have been adopted as the norm many years ago, it being all inclusive of tridactyl theropods while Neotheropoda is not. But no, establishment taxoinertia kept Neotheropoda as the auto go to. 

But that could not last. Eventually an avepod basal to Neotheropoda would pop up. And it is --

Anteavis. 

Which is basal to neotheropods, and metatarsal 1 is a splint too reduced to contact the ankle -- unlike the long stout elements of Eodromaeus and Tawa. 

And naming a node that incorporates Anteavis and the rest of Neotheropoda is still inside Avepoda, and excludes more basal tridactyls. Only Avepoda catches them all (unless metatarsal 1 and tarsals discontact evolved more than once but that is the nonparsimonius hypothesis). 

So we finally have a theropod with a splint inner metatarsal that is longer than those of neotheropods. Let's get our taxonomic butts in gear and start using avepod on a regular basis. 

GSPaul

[Mickey Mortimer]

I was actually thinking of writing the exact opposite comment when I saw the Anteavis paper- regarding how Avepoda is an example of why apomorphy-based clades are bad. Sure the skeletal reconstruction in Figure 1 shows metatarsal I not contacting the tarsus, but the text explains "it cannot be determined if it reached the proximal surface of metatarsal II." So we don't know if Anteavis is an avepod, and one node crownward in their tree we don't know if Lepidus is either. Thus Avepoda has three possible positions in their topology, whereas stem- and node-based clades like Theropoda and Neotheropoda only have one. I'm betting like you that we will eventually discover/recover a tridactyl taxon well supported outside Neotheropoda, but if it's more crownward than Anteavis we'll still have the same problem we do today.

Btw, anyone notice how ridiculously long supposed pedal phalanx I-1 is in Figure 1s? More elongate than any theropod I can think of outside Ornithothoraces. And not commented on by the text, or represented that way in the skeletal reconstruction where it's less than half as long. Of course that reconstruction also gives it four phalanges on pedal digits II and IV...

Mickey Mortimer

[Gregory Paul]

The Anteavis inner metatarsal is a severely reduced splint, and the possiblity of its reaching the ankle is minimal at best. And it is longer than those of neotheropods, and the ilium length is also intermediate to those of neotheropods and four toed theropods. So it qualifies very nicely as a basal nonneotheropod avepod. 

Sticking to node based clades only has its own problems. They always automatically exclude basal members of the clade that have an important anatomical feature of that group, so there is never a node only group that includes all species with that feature. And whether a particular species very close to the base of node based clade belongs in it or just below it can flip as different cladistic results come in. 

The only way to incorporate all monophyletic tridactyl theropods in a clearcut unified clade is by a character plus clade group. If Avepoda is not used, then Anteavis is a nonneotheropod almost but not quite in that clade, as are a number of as yet unknown species lacking inner metatarsal ankle contact. They are higher level taxa orphans. Naming an Anteavis-neotheropod clade solves the problem for that species and those that may or may not be above the node, but still leaves out more basal tridactyl species until yet another lower set node is tagged which itself does not include all three toed species, and so on a perpetual set of taxonomic Chinese boxes that never solve the issue. 

So Avepoda forever! Literally, it will always be there ready to use and incorporate every tridactyl theropod. 

GSPaul

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