It's not every day that we get an article arguing against the dinosaurian origin of birds any more, with that consensus so strongly supported and many of its detractors being deceased. But last week Feduccia published "The Avian Acetabulum: Small Structure, but Rich with Illumination and Questions", so in the spirit of the Dinosaur Mailing List of the early 2000s, let's have a fun critique of it.
Feduccia's attempted main point is an exercise in what Makovicky and Dyke (2001) described as naive falsification over twenty years ago - taking some feature shared by birds and dinosaurs and claiming it's not homologous in those groups, and via a mysterious step 2 concluding phylogeny is in shambles and birds must be non-dinosaurian. Feduccia's (2023) character de jure is the open acetabulum, classic dinosaurian character still seen in birds, and we can use Silesaurus to demonstrate Makovicky and Dyke's entire point that makes Feduccia's argument invalid and useless even if he was correct about the anatomy.
Traditionally an open acetabulum supports dinosaurian monophyly, but I'm a fan of the recent idea that silesaurs are a basal grade of ornithischian and it turns out dun dun DUN silesaurs have closed acetabula. Thus genasaurs and saurischians most parsimoniously evolved open acetabula convergently. According to Feduccia's logic, there goes the idea that ornithischians are closely related to saurischians and archosaurian phylogeny must be reevaulated. But actually no, the vast majority of evidence still strongly supports ornithischians being closer to saurischians than pterosaurs, aphanosaurs, suchians, phytosaurs, etc. because it's not like this example of convergence magically creates evidence ornithischians are related to something else. That would be naive falsification.
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| Top- Figure 3a of Feduccia (2023) illustrating (left to right) Stegosaurus, Allosaurus and 'Postosuchus'. Bottom- actual Postosuchus pelvis after Weinbaum (2002). |
So anyway, did you know "Late Triassic “rauisuchians” like Postosuchus exhibit an entirely theropodan pelvic anatomy, except for the acetabulum, which is largely closed."? Because I didn't. The first thing that's funny is that Feduccia's figure of Postosuchus' pelvis (Fig. 3a) is copied from Figure 15 of Chatterjee's 1985 original description of the genus, and we've known at least since Long and Murry 1995 (TWENTY-EIGHT YEARS AGO) that it's a chimaera using the ilium of Lythrosuchus and the pubis of Shuvosaurus. It's details like this that demonstrate how Feduccia et al. are stuck in the past. In any case, Postosuchus has some very non-theropodish pelvic features like the low ilium, huge supracetabular buttress, only two sacral attachments, non-contacting pubis and ischium, absent obturator flange and the characteristic 'rauisuchian' medioventral tilt, but Feduccia doesn't know this because he's stuck in Chatterjee's 1980s world where Postosuchus might as well be a tyrannosaur ancestor.
Right away we get "An open acetabulum is considered one of the most unchallengeable synapomorphies of dinosaurs and birds as opposed to stem dinosauromorphs and dinosauriforms that had not yet achieved fully upright posture and still exhibit a closed or partially closed acetabulum", but as shown by potentially ornithischian silesaurs, nope. You can go all the way back to Ferigolo and Langer (2007) for the concept, recovered in phylogenetic analyses at least by 2016 by Cabreira et al. and more recently and explicitly argued by Müller and Garcia (2020). The latter don't even mention the word "acetabulum" because modern phylogenetics has long since moved past "unchallengeable" key characteristics.
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| Figure 2 of Novas (1996). |
Most of Feduccia's paper seems to be written in response to a straw man BAD (Birds Are Dinosaurs) scientist imagining "fully open acetabulum" is to be taken literally, having no medial enclosure at all, and don't you know it, that's often untrue in birds and other pennaraptorans. In reality, an "open acetabulum" in Dinosauria is contrasted with having only a narrow slit as in Lagosuchus, Ornithosuchus or indeed as illustrated in Chatterjee's 'Postosuchus' chimaera. Just look at Figure 2 of Novas' classic 1996 work on dinosaur monophyly which shows the dinosaurian perforate acetabulum represented by Herrerasaurus with medial enclosure around the edges just as Feduccia illustrates in so many birds. Even Feduccia's own figure of Stegosaurus' pelvis (which like his Allosaurus pelvis is ultimately copied from Marsh's papers from the 1800s- such a contemporary reference!) meant to show the "open acetabulum in dinosaurs" has a significant medial rim.
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| Meleagris pelvis (top) as redrawn in Baumel and Witmer (1993) from (middle) Harvey et al. (1968), and (bottom) different specimen from Butendiek (1980). |
Amusingly, Feduccia says "The Nomina Anatomica Avium, like most modern avian anatomical references, portrays Gallus (domestic chicken) and Anas (domestic duck) with fully open, dinosaurian acetabula, unlike older images (Figure 1), which show substantial medial acetabular walls." His Figure 1 has drawings of "Gallus, Encyclopaedia Brittanica, 1911; Anas, The Vertebrate Skeleton, Cambridge Press, 1897", which in addition to being downright archaic, are a pop encyclopedia and general vertebrate anatomy book, neither of which seem like somewhere to trust little anatomical details from. Also, the Nomina Anatomica Avium doesn't even figure the pelvis of Gallus or Anas, it has Meleagris (Figure 4.15). And that is credited as being redrawn from Harvey et al.'s (1968) Meleagris osteology, which indeed shows a larger acetabular foramen than Feduccia's 1911 Gallus or 1897 Anas. But maybe it's a fluke. Let's check Butendieck's 1980 Meleagris osteology and nope, the foramen is even larger there. Guess it's not a modern cladist conspiracy.
Feduccia insists on staying behind the times when he writes "The continued illogical application of “phylogenetic nomenclature” [70], “phylonyms” [9], and the arbitrary redefinition of established taxon names necessitates the following nomenclatural clarifications. ... “Archosauria” is also used sensu traditum [71,72] and is therefore equivalent to the Archosauriformes of Nesbitt [73], Ezcurra [74], and de Queiroz et al. [9]" and every other work dealing with archosaur phylogeny and nomenclature since the 90s! Oh, except Benton's (1999) Scleromochlus redescription he cites as reference 72, which proposed the name Avesuchia for the crown group which nobody ever used again. Why Feduccia insists on using a concept outdated by thirty-some odd years and expects to be taken seriously is a mystery. Oh, and in his Table 1 Feduccia defines Ornithurae as "All those birds are more closely related to extant birds (Neornithes) than they are to Enantiornithes", so look who's applying phylogenetic nomenclature now. Guess it can be logical as a concept, who would have guessed?
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| Top left- unfused Meleagris pelvis showing iliac contribution to the acetabulum (25) is larger than ischial contribution (64) (after Butendieck, 1980). Top right- Sinornis pelvis showing antritrochanter (vant) almost entirely on the ilium (after Sereno et al., 2002). Bottom- Buitreraptor (left) and Berlin Archaeopteryx (right) showing antitrochanters (at) (after Agnolin and Novas, 2011). |
It's not just acetabular closure that Feduccia is examining, he also comments on the distribution of supracetabular crests and antitrochanters. For the latter, he notes "A structure homologized with the avian antitrochanter has been reported in ... a miscellany of coelurosaurs [16,102,103]. There is no osteological evidence that these structures are homologous: the avian antitrochanter forms primarily
[69] or almost exclusively [90] from the ischium, whereas the so-called “antitrochanters” of these nonavian taxa are almost exclusively iliac in composition ... Moreover, while continuity of function is not a requirement of homology, it should be noted that putative “antitrochanters” of so great a range of taxa cannot possibly have performed the specialized mechanical role that the true antitrochanter does in extant birds as part of their peculiar hindlimb locomotory system [90,104]." So the antitrochanters of coelurosaurs aren't actually antitrochanters because they're on the ilium! But wait, let's pull out Butendieck's turkey osteology again, and oh no! Meleagris has an antitrochanter mostly formed by the ilium! And while Feduccia says "In both enantiornithines and ornithurines, the maintenance of balance in obligately bipedal locomotion is linked to the presence of an antitrochanter", Sereno et al. (2002; in a figure Feduccia specifically cites) shows that in the enantiornithine Sinornis the antitrochanter is almost entirely on the ilium despite supposedly having the same function as in living birds! That's also true in the Lecho Formation enantiornithine pelvis PVL 4042 by the way (Walker and Dyke, 2010: Fig. 14). Given even those three examples, when Feduccia says "The putative “antitrochanters” of alvarezsaurs and therizinosauroids are iliac in composition", or that in Rahonavis "the purported antitrochanter, however, is iliac in origin", it lacks any force of argument whatsoever. Since it CAN be that way in birds and even supposedly have the same function, it can also be that way in bird ancestors and relatives.
Similarly, for Archaeopteryx Feduccia claims "Agnolin and Novas [115] (Figure 3B) interpreted a slight swelling on the posteroventral rim of the acetabulum in the Berlin specimen as an antitrochanter, but this interpretation seems incorrect because (1) it is positioned on the posteroventral rim of the acetabulum, whereas the avian antitrochanter is posterodorsal to the acetabulum [69,90,104]; it is composed solely of the ilium, whereas the avian antitrochanter is composed exclusively or principally from the ischium." But as the ischial peduncle doesn't wrap beneath the acetabulum, the structure is clearly on the posterodorsal rim, and is furthermore basically identical in structure and position to that of Sinornis (and Buitreraptor shown next to it, contra Feduccia's claim unenlagiids lack antitrochanters) except for being a bit more slender than in Sinornis. How Feduccia can have looked at these figures and think Sinornis has an antitrochanter but Archaeopteryx does not is beyond me. Another example of Feduccia's confusion is when he states "Avimimus portensosus [sic], however, possesses an ischial antitrochanter that, therefore, appears homologous with that in birds" but then notes A. nemegtensis was diagnosed in part by Funston et al. (2018) as lacking an antitrochanter. Yet Funston et al.'s same figure cited by Feduccia shows the antitrochanter in A. portentosus is largely on the ilium (as Figure 4G doesn't even show the ischium) and indeed, A. nemegtensis doesn't preserve a proximal ischium at all so if the antitrochanter were ischial neither Funston et al. nor Feduccia would even know it lacked one! The basic conclusion is Feduccia doesn't know what he's looking at, making his attempted review of antitrochanter distribution useless.
Feduccia is no better when discussing supracetabular crests, as evidenced when he says "Although Paul (2002) claims that a supracetabular crest is present in archaeopterygids, it is absent in the London, Berlin, Eichstatt and Munich specimens (Figure 8d)." But Paul's Plate 24Ab clearly shows a convex outer rim in ventral view that defines a supracetabular crest in a photo of a cast of the London specimen, whereas Feduccia's Figure 8d is a horizontally oblique drawing of a reconstructed generic Archaeopteryx pelvis that couldn't show lateral convexity in dorsal/ventral view even if it was there. So far from countering Paul, the medium is inferior, the subject is inferior, and the perspective would make evaluation impossible in any case. Indeed, Paul's entire Plate 24 is an effective argument against everything Feduccia claims about antitrochanters and supracetabular crests, showing for example that tyrannosaurids lack a supracetabular crest, so apparently it wasn't integral to a functioning theropod gait, and that young Meleagris lacks an ossified antitrochanter, so maybe at least some of these theropods with seemingly small or absent antitrochanters had larger cartilaginous structures that enabled them to be functionally similar to living birds and/or Avimimus portentosus.
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| Right- Feduccia's (2023) Figure 5 (caption below). Top left- Megapnosaurus rhodesiensis pelvis (after Raath, 1977- see, I can cite old papers too!). Bottom left- Oksoko pelvis (flipped horizontally, after Funston et al., 2020). |
My favorite part of this paper though is Figure 5, which shows drawings of the "Pelvis of the basal theropod Coelophysis (a) compared to an array of primarily ground-dwelling birds: (b) Solitary Tinamou (Tinamus solitarius), (c) Wild Turkey (Meleagris gallopavo), (d) Japanese Quail (Coturnix japonica), (e) Greater Roadrunner (Geococcyx californianus), (f) Emu (Dromaius novaehollandiae), (g) Bush Moa (Anomalopteryx didiformis), and (h) Elephant Bird (Aepyornis hildebrandti)." The first thing of note is that the Coelophysis drawing is highly inaccurate, being a copy of Colbert's (1989) illustrations by Lois Darling, which "do not depict the anatomy of Coelophysis accurately" (Downs, 2000). There are no pubic or obturator foramina, no obturator plate on the ischium, the supracetabular crest is not confluent with the ventral postacetabular edge, and rather importantly for this paper's subject the ilium is lacking the medial wall on the dorsal part of the acetabulam (that is partially ovelapped laterally by the supracetabular crest) and the large antitrochanteric surface formed by the ischial peduncle and ischium. While I agree with Feduccia the latter structure is not homologous to the maniraptoran antitrochanter (which is on the outside rim of the acetabulum), it does combine with the dorsal section to make a significant surface medial to the acetabular rim. Again, Feduccia is using a 30+ year old source that is well known to be wrong.
And what's even the point of the figure? "Note the contrast between the condition in these cursorial birds, which presumably should most closely approach the putatively ancestral theropod condition with respect to their acetabular morphology, and the condition in Coelophysis." First of all, why should an avian/neornithine anatomy that evolved in the Late Cretaceous converge with or regress to a Triassic group's morphology? Second, modern birds have a functionally different gait from non-maniraptoriform theropods like coelophysoids at least, where the femur is more horizontal and less mobile, so even in terrestrial birds I don't see why any special acetabular anatomy would be similar. Third, Feduccia thinks oviraptorosaurs are birds (e.g. Feduccia and Czerkas' 2015 "Testing the neoflightless hypothesis: propatagium reveals flying ancestry of oviraptorosaurs") and oviraptorids like Oksoko happened to have an even more open acetabulum than Darling's fake Coelophysis and a pelvis much more superficially similar to coelophysoids than to modern birds. So according to Feduccia, some cursorial birds DID approach what he imagined the ancestral theropod condition to be like. He just chose to only figure cursorial crown birds, which aren't very similar to coelophysoids in pelvic structure.
Looks like my work in the Lori description (Feduccia's reference 52) got cited! "Reliance on simple tabulations of step count difference between constrained and unconstrained trees [52] to determine whether the most basal members of pennaraptoran clades have been correctly resolved by parsimony analysis of current data matrices is naive and takes no account of the statistical significance of differences in tree populations [50]." Oh noes! I was naive! And what's this reference 50 I should have taken into account? James and Pourtless 2009! The study that actually found birds as maniraptorans, coelurosaurs, theropods and dinosaurs, but which hid it by only figuring majority rule bootstrap trees and always pruned out Effigia because it falling out as an ornithomimosaur was too embarrassing of a result even for them! And that was even after scoring all dinosaur manual characters as unknown, even in the pentadactyl Eoraptor and Herrerasaurus with zero controversy of digital homology! Bwa ha ha! Yeah, those are some methodologies I really should have taken to heart. Hey, at least Feduccia cited me as the correct author this time!
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| Top- Ornitholestes holotype ilium and proximal pubis (after AMNH's 2007 digital collections). Middle- Allosaurus USNM 4734 ilium (after Gilmore, 1920). Bottom- Tyrannosaurus FMNH PR2081 ilium and sacrum (after Brochu, 2003). Note even the latter classic theropod has significant medial walls on the peduncles. |
But what does any of this have to do with birds not being dinosaurs? Feduccia spends lots of time discussing the amount of acetabular closure in modern birds, always emphasizing the medially walled areas, and going so far as to say "it is more difficult to adequately assess acetabular morphology in extant birds than is generally realized. Preparatory methods used to produce skeletal collections in museums (e.g., defleshing with dermestid beetles, oxidation methods involving HO, and ammonia, chemical bleach, and maceration) can damage or destroy fragile anatomical features" so that we can't even trust our own specimens! It's as if in his typological mind, this is making birds "less dinosaurian" because dinosaurs are things with open acetabula, and shouldn't this be concerning to us BADists?! No. No it should not. Hell, all crown birds could have fully closed acetabula like derived ankylosaurs and it wouldn't matter one bit because key characters aren't a thing and evolution happens. Just like it doesn't make ankylosaurs any less dinosaurian. We wouldn't even need the fact that every single stem bird fossil* has an open acetabulum regardless of preservation, and that makes it undeniable that regardless of the condition in crown birds, and regardless of what birds evolved from, Mesozoic birds have open acetabula.
* Even scansoriopterygids which he lies about, claiming "they have closed acetabula" when even his previous description with Czerkas (2014) cited "the partially open acetabulum in Scansoriopteryx." Czerkas and Yuan's (2002) original description also states "the indication from the texture and color extending from the ilium suggests that the hip socket was not as widely perforated as in theropods or dinosaurs in general. The inner edge of this reduced perforation in the hip socket can be seen in both acetabula on the counterslab."
Feduccia will emphasize "partially closed" in many examples, but non-pennaraptoran theropods are not the monolith he takes them for when as in 2014 he and Czerkas write "theropod dinosaurs, invariably exhibiting a completely perforated and open acetabulum", or as he says in the present paper "The condition in the large Jurassic tetanurine Allosaurus (Figure 3) may be taken as stereotypical of its development in Theropoda", with Figure 3 being redrawn from the 1880s. Just check out classic theropod Ornitholestes above with a medial wall taking up almost half of the acetabular circle. Even in Allosaurus itself, if you look at an actual photo you can see that Figure 3's seemingly uninturrupted dorsal edge is wrongly drawn and that half the ischial peduncle is medial as is some of the pubic peduncle but most of that is hidden by the laterally overlapping supracetabular crest except for the posteroventral corner. Dinosaurs all over the cladogram were changing the amount of medial acetabular ossification and birds were no exception.
And really, the takeaway I got from Feduccia's data is that just like most characters, the development of acetabular closure, supracetabular crest extent and antitrochanter size are labile and prone to homoplasy. They are not integral parts of a locomotory system, and similar taxa can manage movement just fine when these variables are changed. That's why you can have Avimimus portentosus and A. nemegtensis with such drastic differences in antitrochanter development, which Feduccia seems baffled by- "it is not clear why a single genus should be polymorphic for this character state, it is difficult to interpret this character conflict." Similarly Feduccia says "It is surprising, given the
frequent characterization of birds as having — like dinosaurs — completely open acetabula to
discover that rotisserie [chicken] specimens have partially closed acetabula with a perforation covered
by soft tissue." Are we assuming Mesozoic dinosaur acetabular foramina were not covered by soft tissue? And is there really a functional difference between the paper thin medial bone walls Feduccia is worried might be present in modern birds but destroyed by preparation and a fibrocartilage wall hypothetically present in a theropod with an open bony acetabulum? Based on Feduccia's described variation within modern birds, I'm doubtful. But he's so obsessed with these being typologically enforced key characters essential to function and evolutionary history, for both himself and his BAD straw man, that them being not all that important in anathema to him.
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| Now, this is out of order but I wanted another figure to liven up the post. On the left is "Late Jurassic Archaeopteryx (image composites of varied sizes, primarily after P. Wellnhofer, based primarily on the Munich specimen, with details from the London, Berlin, and Eichstitt specimens;". Yeah, there is no closing parenthesis and I think "image composites of varied sizes" refers to the Gansus, Qiliania and Pterygornis pictures featured elsewhere in this figure. More importantly as noted far above, this figure is just a drawing (by K. Grow apparently) of a hypothetical composite, and while it is used as an argument against a supracetabular crest in the genus it is incapable of showing the development of a supracetabular crest directed at the viewer as in e.g. Ornitholestes. On the right is an item labeled "Microraptor, the Early Cretaceous four-winged bird-like glider (dromaeosaur), considered by many an early bird. (Microraptor photo courtesy D. Burnham)", but some digging finds this is actually the structure labeled "Right lateral view of microraptoran pelvis (modified cast)" in Burnham's (2007) thesis. The latter claims it is "based on a recent transfer preparation of a Chinese microraptorine, which exposes a lateral view of the acetabulum." Who knows what parts were modified and we have no traceable specimen to confirm anything with, although the preacetabular process and ischial tip don't look like those in actual Microraptor fossils. I'm not even implying any purposeful anatomical inaccuracy, just noting that Feduccia passed off a "modified cast" as an apparently real specimen. |
In the end, Feduccia admits that if "the first birds — and at least taxa like the microraptorines — were not terrestrial cursorial bipeds, like theropods, but trunk-climbing gliders" it "would be consistent with arguments that (at least some of) these taxa are “neoflightless” theropods, whose immediate ancestors underwent transformations in the theropod locomotory system (perhaps because of a shift to arboreality)." But he rejects this, in part due to "the discovery of a “tetrapteryx” stage in early aviation evolution", which is a Petersian thing to say because not even the relatively well resolved pennaraptoran consensus phylogeny views the tetrapteryx stage as an established thing, let alone Feduccia's wacky world of uncertainty where who knows how oviraptorosaurs, troodontids, eudromaeosaurs, microraptorians, unenlagiids, anchiornithines and archaeopterygids interrelate. Sure microraptorians have big leg wings, and Sapeornis and anchiornithines have a much shorter version, but the former are dromaeosaurids and the latter are maybe archaeopterygids, maybe avialans, maybe troodontids. The structures are lacking in Caudipteryx, troodontids, Archaeopteryx and pygostylians, so in order for a tetrapteryx stage to exist, dromaeosaurids and/or anchiornithines would have to be ancestral to crown birds somewhere or we have a lot of reversals. Note Feduccia claims "if Jinfengopteryx is, in fact, a basal troodontid [140,236] (but see discussions in [115,139,162]) then troodontids also primitively exhibited a “tetrapteryx” bauplan", but Jinfengopteryx has no long leg feathers at all, not even long tibiotarsal ones like Archaeopteryx and basal pygostylians. So that's just him being wrong again.
And in part the rejection is due to... it's really not made clear. Feduccia assumes us BADists are as constrained by imagined evolutionary just-so stories as he is, claiming we are bound to "the argument — advanced since Huxley, championed by Ostrom, repeated ad infinitum — that the obvious reason birds are obligate bipeds and that they never passed through an evolutionary phase in which all four limbs were integrated into the flight mechanism, is that birds are descended from obligate bipeds, viz. theropods." But he already said a facultatively bipedal stage (around the paravian stem maybe?) due to arboreal habits would still be consistent with a theropod ancestry, as quoted in the prior paragraph. Feduccia continues "These data instead suggest that the reason bipedalism in birds differs fundamentally from bipedalism in all other archosaurs [does it?] ... is that birds started their evolutionary trajectory toward obligate bipedalism from a different starting point than the system employed to such success by dinosaurs." But poposaurs and dinosaurs (or alternatively saurischians and Lesothosaurus+genasaurs) started their obligate bipedalism trajectory via facultative bipeds with a more sprawling stance, more closed acetabula, no antitrochanter and less inturned femoral heads just as Feduccia believes birds did. It's just special pleading to claim they are "fundamentally different." Finally, he claims "This is a logical explanation for the surprising persistence of medial occlusion of the acetabulum in many pennaraptorans and the absence, in most, of either well-defined supracetabular crests or antitrochanters." This despite explicitly saying two pages ago "Oviraptorosaurs uniformly display fully perforate acetabula" and "Troodontids have fully perforated acetabula", so by "many pennaraptorans" Feduccia really just means dromaeosaurids. He also said on the previous page "Compsognathids are “prototypical” small theropods [221], yet they lack a supracetabular crest or antitrochanter", so why would this be surprising in most pennaraptorans? (lacking a hypertrophied antitrochanter like ornithurines at least...)
Amusingly enough, the most vehement objections to BAD are in Feduccia's Figure 7 caption showing hesperornithid pelves. This claims without supporting data "the acetabula show that they are definitively not allied with dinosaurs" and "Hesperornithiforms are often termed diving dinosaurs, but given their acetabula anatomy this designation is erroneous." I'd think such late and specialized divers would be of little relevance, but what do I know... Hey that long and low postacetabular process is looking pretty similar to Postosuchus. Maybe we've found our archosaurian avian ancestor ;)
And that's it. Feduccia's Conclusion is "The hypothesis that birds are maniraptoran theropod dinosaurs, despite the certitude with which it is proclaimed, continues to suffer from unaddressed difficulties [29-31,50,237]", with references 29-31 being his own books and paper, 50 we recall is James and Pourtless' terrible TERRIBLE attempt at a cladistic analysis, and 237 is James' fawning review of Feduccia's latest book. Ha. You know what's an unaddressed difficulty in Feduccia's hypothesis? Thirty-plus years since he publically doubted the dinosaurian origin of birds, we still have pennaraptorans emerging from some mysterious lineage of Triassic archosaur(-iform)s. And yet we've closed the gaps so well in theropod phylogeny that the precise relationships among basal avialans, basal paravians, basal pennaraptorans, basal maniraptorans/-iforms, basal neocoelurosaurs, basal coelurosaurs, etc. are controversial due to homoplasy. Surely any day now we'll find the evolutionary intermediates between Middle Jurassic scansoriopterygids and Triassic ???????. Anything But a Small Running Dinosaur indeed.
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