Mexidracon longimanus, new ornithomimid from Upper Cretaceous of Mexico

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

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Mexidracon longimanus gen. et sp. nov.

Claudia Inés Serrano-Brañas, Belinda Espinosa-Chávez, Claudio de León-Dávila, S. Augusta Maccracken, Daniela Barrera-Guevara, Esperanza Torres-Rodríguez & Albert Prieto-Márquez (2025)
A long-handed new ornithomimid dinosaur from the Campanian (Upper Cretaceous) Cerro del Pueblo Formation, Coahuila, Mexico
Cretaceous Research 106087
doi: https://doi.org/10.1016/j.cretres.2025.106087
https://www.sciencedirect.com/science/article/abs/pii/S0195667125000102


Highlights

Ornithomimosaur remains from the Campanian Cerro del Pueblo Formation are described.
The morphological characteristics of the specimen indicates the presence of a new taxon.
The most conspicuous characteristic is the exceptionally enlarged metacarpals.
Mexidracon longimanus is the first ornithomimid formally described from Coahuila.
This study points to a greater diversity of ornithomimosaurs in southern Laramidia.

ABSTRACT

New ornithomimosaur material discovered from the Upper Cretaceous Cerro del Pueblo Formation (CdP) of Coahuila, Mexico represents a new genus and species of Ornithomimidae. The new taxon, Mexidracon longimanus sp. nov., is represented by an individual preserving axial and appendicular elements. M. longimanus is characterised by the following combination of characters: extreme lengthening of the metacarpals that are longer than the metatarsals, proximal end of metacarpal II with a narrow subtriangular outline, a pubic peduncle of the ilium with a flared, zig-zag articular margin that is wider anteriorly than posteriorly and an ischiadic peduncle that is similar in size to the pubic peduncle, a pubic boot where the distal margin of the anterior expansion is separated from the shaft by a deep notch, a femur that is slightly longer than the tibia, an arctometatarsalian pes, a metatarsal II that has a D-shaped cross-section, and a metatarsal IV longer than metatarsal II, among other features. A phylogenetic analysis places M. longimanus within Ornithomimidae forming a polytomic relationship with other members of this clade. The finding of M. longimanus adds to the increasing diversity and paleobiogeographic distribution of the group during the Campanian of southern Laramidia. The ornithomimosaur record of the CdP represents yet another instance of the coexistence of ornithomimids and deinocheirids spanning a wide range of body sizes within this clade of ‘ostrich’ dinosaurs.

[Mickey Mortimer]

So I'm finally getting around to looking at this taxon, and is it just me, or are the supposed long metacarpals just the right metatarsus instead? The metacarpus (mcII) is supposed to be 124% the length of the metatarsus (mtIV), which in something with long metatarsi like an ornithomimid is just ridiculous. For comparison, that ratio is 28% in the already long-handed Dromiceiomimus, 25% in Anserimimus, 23% in Gallimimus, 30% in Struthiomimus, 17% in Sinornithomimus. Even in something with a stubby pes like Deinonchieus this ratio is ~45%, and the pes in Mexidracon is pretty slender. Sure we're getting some weird dinosaurs lately, but can you really picture an ornithomimid with a hand over four times longer than all the others, despite having a normal-sized humerus (66% of femoral length vs. 65%, 60%, 80%, 65%, 66% and 75% respectively in the ornithomimosaur taxa listed above)? 

And if you look at the proximal supposed metacarpals I and II, it's just the shape of a theropod metatarsus proximally (with mtIII proximally fused), whereas no ornithomimosaur has a metacarpal I proximal end over twice the area of metacarpal II, with a weirdly lenticular metacarpal II that lacks a broad dorsal edge (just look at their own Figure 19). Similarly, the distal ends of supposed metacarpals II and III are just the right shape for e.g. Gallimimus distal metatarsals III and IV respectively (Osmolska et al., 1972: Fig. 16B). Actual ornithomimid metacarpals have deep ventral grooves and taper dorsally in distal view (Osmolska et al., 1972: Fig. 13B). The authors say "The mid-shaft of MCII is triangular in cross-section", which is not typical of theropod metacarpals but is the shape of arctometatarsalian third metatarsals. Finally, the supposed metacarpus is associated with the right tibia and fibula.

Of course a right metatarsus 24% longer than the left is odd as well, but the bones are all fractured, both supposed metacarpals I and III lack their ends, and my identifications would match up supposed distal metacarpal II to part of supposed proximal metacarpal I, so the continuity of fractured pieces would be illusory anyway. The supposed distal end of metacarpal I works just fine as a displaced distal metatarsal II, while the supposed "first half of the shaft of MCIII [which] closely adheres to MCII on its lateral side" could easily be the wall of the right fibula, with the right fibular fragment in Figure 7G being part of metatarsal V.

A reviewer must have brought this up because the authors have a few paragraphs addressing it on page 13, but their "Size" section is irrelevant because I don't identify either proximal end as completely metatarsal III, and ditto the second paragraph and first part of the third because I'm not equating the supposed proximal end of mcII to mtIII. They then say "the mid-shaft cross-section is different between metacarpals and metatarsals (with the exception of MCII and MTIII which are both similar in shape). In the case of MCI and MCIII, both elements have oval cross-sections, whereas MTII and MTIV have D-shaped and subtriangular cross-sections, respectively", but I'd reply that cross sections can vary throughout the shaft (just look at the confusion that's caused in caenagnathids) and that difference is still less weird than all the others I've noted even if true. Finally, they claim metatarsal II has a different distal cross section compared to supposed metacarpal I (which is stuck to the femur so not really visible in distal view), but I'd say the rounded shape matches a metatarsal II much more than a metacarpal (which again, are V-Y shaped in distal view) and the sharp drop shape of left metatarsal II is potentially caused by breakage and/or exposure.

Mickey Mortimer

[Thomas Richard Holtz]

You are, in my opinion, correct on the anatomical identity. Indeed, immediately after the online publication, several of us commented on this. A response paper is in the preliminary stages.

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[Milo Gaillard]

I see. It seems like some of the anatomical details may have been misinterpreted according to you.

Question: does this result in Mexidracon being a nomen dubium, or is it still diagnostic? Just curious.

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[Thomas Richard Holtz]

So long as parts of the fossil are diagnostic relative to other species, the taxon remains diagnostic. Misinterpretation of one part doesn't invalidate the whole thing.

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[Milo Gaillard]

Thank you. I was asking because I know it can, depending on the parts being identified & the material being that is worked with.

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[Mickey Mortimer]

Hey, good to know others including Tom agree. Very vindicating.

As for Milo's question, the diagnosis minus the manual characters is-

"(1) elongated distal dorsal
vertebrae; (2) distal dorsal, sacral and caudal vertebrae without
pneumaticity and lacking keeled ventral surfaces; ... (9*) iliac pubic peduncle with flared, zig-zag shaped
articular margin that is wider anteriorly than posteriorly; (10) ilium
with ischiadic peduncle slightly longer proximodistally than pubic
peduncle; (11) deeply concave iliac brevis fossa that is expanded
posteriorly and overhangs lateral surface of postacetabular process;
(12*) distal end of pubis with distal margin of anterior expansion
separated from shaft by deep notch; (13) femur slightly longer than
tibia; (14) accessory trochanteric crest on distal end of lesser
trochanter; (15) distally conical lateral condyle of femur; (16) arctometatarsalian
pes; (17) distal tarsal 3 with nearly straight
posterolateral edge and convex medial margin; (18) trapezoidal
distal tarsal 4 with lateral surface lacking anterolateral projecting
flange; (19) metatarsal II with D-shaped cross-section; and (20)
metatarsal IV longer than metatarsal II."

Of these, at least 1, 2, 11, 15, 16 and 20 are typical of ornithomimids. I'm skeptical 12 is typical anatomy, even though the authors say "The margin along the notch is intact and shows no evidence of remodeling, sclerotization, erosion, or bone overgrowth, so it is not the result of fracture or pathology", because you just don't see notches like that in pubic or ischial feet and it makes no sense. If it really isn't taphonomic or pathological, maybe it's developmental. 13 would also be weird for any small coelurosaur and both femora have completely reconstructed sections of shaft so may have actually been shorter. Finally, I don't see an accessory trochanter in the left femur (Fig. 10H) and I doubt the anterior outline on the right one (Fig. 11H) is natural external bone edge because it's shaped so weirdly and doesn't match the other femur, projecting anteroventrally, so there goes 14 (though even if present, it is normal for ornithomimids though not universal- e.g. absent in Anserimimus and sedens). Based on the authors' own Figure 21, character 17 seems the same as Garudimimus, Struthiomimus and the Scollard Formation RAM 6794. I'd say 18 is also basically like RAM 6794, though that's just an unnamed specimen and I'd be willing to include it in a list of unique character combinations to support validity of Mexidracon. So that leaves 9, 10, maybe 18 and 19 as potentially valid characters which I haven't looked at yet.

Mickey Mortimer

[Milo Gaillard]

Thank you

Sent from my iPhone

On Aug 31, 2025, at 05:56, Mickey Mortimer <therizino...@gmail.com> wrote:



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