In the previous article we looked at the Birds Come First, or BCF, hypothesis. It goes without saying that BCF has not won acceptance in the community, nor – in fact – is it even familiar to the majority of archosaur workers. Here, we take a critical view of BCF: does it stand up, or does it fail?
To begin with, let me note that the underlying premise of BCF is fundamentally problematical: the idea that there is some sort of ‘central trunk’ (Olshevsky 1994, p. 42) or single, unbroken lineage that extended from the first stem-archosaur to the most recently evolved, modern bird. Old ‘family tree’ diagrams did indeed show a single, central lineage that included the ancestors of the various ‘branches’ – this is where we get the erroneous idea of ladder-like progression and missing links from – but this concept is erroneous and always relies on the idea that one group or species is somehow special, or sat at the ‘top’ of the tree (in most trees that ‘special’ organism is Homo sapiens). It’s true that birds represent a highly speciose clade relative to other archosaur groups, and that – relative to the ancestral condition – they are one of the most ‘modified’ archosaur groups, but they are no more ‘central’ than any other lineage: they are simply one side-branch among many. It’s misleading to imagine that all other archosaurs branch off of the lineage that was destined to lead to birds.
Furthermore, if we indulge in a bit of concestor reconstruction, we see no indication of small, arboreal ancestors at any of the divergence points within non-paravian archosaurs. To take a few examples: the concestor of the coelophysoid + other theropod clade was apparently a small, terrestrial carnivore that resembled Coelophysis, the concestor of the sauropodomorph + theropod clade was a terrestrial, presumably bipedal omnivore that looked something like Thecodontosaurus, and the concestor of the ornithischian + saurischian clade was a bipedal, terrestrial omnivore or carnivore with grasping hands and good cursorial abilities, apparently something like a cross between Eoraptor and Heterodontosaurus. And so on.
Despite this, I think that there are a few aspects of BCF that do have an intuitive appeal. The notion that the gradually accrued, bird-like characters of theropods might have appeared as ‘improvements’ for an arboreal way of life does seem fairly logical, for example. But, ultimately, BCF is entirely unsatisfactory: we’re supposed to construct scientific hypotheses based on the evidence we have, rather than on the evidence we think there should be, and BCF is just way too speculative. It proposes the existence of a whole lineage of hypothetical creatures that are absent from the fossil record.
Where are the dino-birds?
There is, as yet, no evidence that the dino-birds predicted by BCF ever existed. The existence of such creatures is pretty important for the hypothesis: the discovery of a small, arboreal, feathered maniraptoran theropod would not provide obvious support for BCF given that such creatures are entirely compatible with the standard model. Look at the scansoriopterygids, for example: they are deeply nested within the maniraptoran radiation, and do not tell us anything about the animals along the theropod or dinosaurian or archosaurian stem. Conversely, the discovery of a small, arboreal basal ornithischian, or crurotarsan, or stem-archosaur would provide support for BCF, yet the fossils we have so far are entirely consistent with the mainstream view that the ancestral members of most archosaur clades were reasonably large (as in, more than a few kilos at least), terrestrial animals without climbing specialisations.
A few fossils have been suggested to be potential dino-birds, but they’re nothing to do with dinosaurs, nor even with archosaurs. Megalancosaurus from the Late Triassic of northern Italy was used as a dino-bird by Olshevsky (1994). It’s a weird climbing beast with a prehensile tail, described by Renesto (2000) as having a ‘bird-like head on a chameleon body’, and it doesn’t exhibit any dinosaurian characters: it’s part of a very weird group of diapsids (the drepanosaurids) that differ fundamentally from dinosaurs, and indeed from ornithodirans and from most archosaurs, in most aspects of anatomy [adjacent Megalancosaurus reconstruction © Silvio Renesto, from S. Renesto's Vertebrate Paleontology at Insubria University site, used with permission].
Authors have generally considered drepanosaurids to be close to protorosaurs (the archosauromorph clade that includes Tanystropheus and kin; Archosauromorpha is the diapsid clade that includes protorosaurs, rhynchosaurs and archosaurs: for discussion please see the third Tet Zoo rhynchosaur article). However, Senter (2004) proposed that drepanosaurids are rather more basal within Diapsida than this, and that – together with Longisquama and Coelurosauravus – they belong to a new clade (dubbed Avicephala) that is only one step up on the cladogram from Petrolacosaurus and kin, the most basal diapsids. If Senter is right, then drepanosaurids have nothing whatsoever to do with archosaur or archosauromorph evolution.
However, I’ve always been pretty sceptical of the whole Avicephala thing, mostly because I don’t find the evidence linking Longisquama and Coelurosauravus with the drepanosaurids to be convincing. Renesto & Binelli (2006) looked anew at Senter’s data set and included lots of new data from additional drepanosaurid specimens. They also included a basal pterosaur. They found a pterosaur + drepanosaurid clade to be within Archosauromorpha, closer to Archosauria than to Protorosauria (pterosaurs are not archosaurs within this phylogeny). Meanwhile, Longisquama and Coelurosauravus remained allied, and in the position that Senter recovered for Avicephala, but nowhere near drepanosaurids [a very simplified version of Renesto & Binelli's cladogram is shown above]. However, even if pterosaurs and drepanosaurids do form a clade, there seems no obvious reason why the features of their common ancestor also applied to the archosaur common ancestor: none of the basal archosaurs or the basal members of the archosaur out-groups (like Prolacerta, protorosaurs and rhynchosaurs) were tiny, climbing animals. Instead, they were mid-sized, terrestrial animals, perhaps able to clamber about in shrubs and whatnot, but without arboreal specialisations (though it’s been suggested that some trilophosaurids were scansorial).
Longisquama – famous for its amazing dorsal ‘plumes’, likened (erroneously) by some to feathers – has also been imagined as a dino-bird, but, as will already be clear, it shouldn’t be linked with dinosaurs or even archosaurs. It exhibits no dinosaurian characters, and doesn’t even seem to be an archosauromorph, despite thoroughly unconvincing efforts to show that it has an antorbital fenestra and might resemble birds (e.g., James & Pourtless 2009). Cosesaurus [shown here] – a small, quadrupedal reptile from the Triassic of Spain – was also suggested by Olshevsky (1991) to be a potential dino-bird, and spike-like impressions preserved on either side of its tail were even suggested to be proto-feathers. It’s now agreed that Cosesaurus is a protorosaur (Peters (2000) interprets it as a close relative of pterosaurs, with both taxa being nested within Protorosauria), and the impressions on the tail are apparently sedimentological artefacts (Ellenberger & Villalta 1974, Ellenberger 1977). As such, Cosesaurus does not have any special relevance for archosaur or dinosaur evolution, and (like other protorosaurs) it lacks any obvious indication of an arboreal lifestyle.
In short, there is no evidence that small, arboreal non-maniraptoran archosaurs might have existed. There are no ‘dino-birds’.
Finally, what of the ‘three problems’
We saw in the previous article that BCF posits the existence of three problems that supposedly show how the standard hypothesis of archosaur phylogeny is flawed and inferior to BCF. In fact, none of the problems really are problems.
The ‘time problem’ points to the fact that basal birds (Jurassic archaeopterygids) predate the flightless maniraptorans that are, according to the standard phylogeny, more basal than birds. Ergo, flightless maniraptorans (the mostly Cretaceous dromaeosaurids, oviraptorosaurs and such) are more likely derived from archaeopterygid-like ancestors, rather than vice versa [as shown in the cladogram below. Illustrations by Ken Kirkland (troodontid), Darren Naish (oviraptorosaur) and Greg Paul]. Several observations nullify this contention. Firstly, we construct phylogenetic hypotheses by looking at the distribution of characters: the distribution of taxa within time is effectively irrelevant. It doesn’t matter that some maniraptorans are geologically younger than the oldest known birds: phylogenetic analyses that have good sampling across taxa and morphology still show that those maniraptorans are more basal than birds. Secondly, it is dishonest to imply that the absence of some maniraptoran lineages from Jurassic strata is really that meaningful: for reasons that are not well understood, the Jurassic record of small theropods is poor, and the few taxa that are known from good remains generally come from rare lagerstätten deposits. Thirdly, despite this poor Jurassic record, some Jurassic fossils are compatible with standard phylogenies. Eshanosaurus is a possible therizinosauroid from the Early Jurassic (see the comments here for more), and it’s no secret that a troodontid is now known from the Morrison Formation, for example. Numerous dromaeosaurid and additional troodontid fossils (admittedly, mostly consisting of teeth) are similar in age to archaeopterygids, showing that deinonychosaurs are at least as old as birds. These discoveries – Eshanosaurus in particular – indicate long ghost lineages for many maniraptoran clades. In other words, there is no ‘time problem’, just a spotty fossil record.
The ‘size problem’ asserts that the evolution of small size among proto-bird maniraptorans is, in the standard model, incompatible with Cope’s Rule and hence less likely than the alternative (that small size was present ancestrally throughout the history of Archosauria, and that large size is always derived). The fact is that lineages can become miniaturised during evolution, though the reasons for such modifications necessarily remain speculative. Paravians evolved small size (Turner et al. 2007), and it’s possible that this has something to do with climbing, with flight, or with something else entirely (like avoiding predation from other theropods, or exploitation of particularly small prey). All of the really small coelurosaurs that are now known (they include the dromaeosaurids Mahakala, Shanag, Rahonavis and Microraptor, the troodontid Jinfengopteryx, the scansoriopterygids, and a few alvarezsaurids and oviraptorosaurs) are maniraptorans, and there is no evidence that small size evolved further down the tree. In short, we simply have to posit that small size really did evolve somewhere within Maniraptora, Cope’s Rule or not. In other words, there is no ‘size problem’, just a single evolutionary event.
Finally, the ‘wing problem’ argues that the standard model provides no obvious explanation for the evolution of the avian wing, and that the wing most likely evolved in an arboreal setting among dino-birds. The fact that long remiges have now been documented in oviraptorosaurs, dromaeosaurids and other maniraptorans shows that feathered arms essentially the same as those present in basal birds evolved somewhere round about the base of the oviraptorosaur + paravian clade, and there is no evidence that wing-like arms were present in more basal coelurosaurs, nor in other theropods, or other dinosaurs, or other archosaurs. Why did long remiges evolve among maniraptorans? We don’t know. One proposal is that enlarged remiges helped small maniraptorans to propel themselves up steep inclines and tree trunks: so called flap-running, or wing-assisted incline running, or WAIR (Bundle & Dial 2003, Dial 2003a, b, Dial et al. 2008) [adjacent image shows WAIR in action. Even 'half a wing' is indeed useful]. Fact is, recent discoveries have shown that the avian wing is really nothing special compared to the forelimb morphology present elsewhere in Maniraptora, and the ‘wing problem’ wrongly assumes that it is. Ergo, there is no ‘wing problem’.
Wrapping it all up
BCF is a very interesting hypothesis, not least of all because it proposes the existence of numerous small, climbing archosaurs – those ‘dino-birds’ – that sound fun and interesting. But it fails on many levels. (1) It assumes that all of the bird-like characters that accrued during archosaur evolution accrued because the clade as a whole was heading toward increasing birdiness, but this is only true if you imagine to start with that birds have a special place on the cladogram. There’s no reason to think that this is true. (2) It assumes that the concestors all the way along the archosaur cladogram were small, bird-like, arboreal animals, yet there is no evidence for this, and outgroup comparison always refutes it (except, notably, within Maniraptora). (3) It posits the existence of numerous hypothetical ancestral ‘dino-bird’ forms, yet these remain hypothetical and all purported ‘dino-birds’ have turned out to be something else. (4) It states that the evolution of small size and evolution of wings within proto-birds, and the stratigraphical distribution of maniraptorans, are unexplained by conventional phylogenies, yet they are clearly not problematical.
For previous articles on non-standard phylogenetic hypotheses see…
- Goodbye from the stem-haematotherm, goodbye from me
- Aquatic proto-people and the
theoryhypothesis of initial bipedalism
- Amphisbaenians and the origins of mammals
And for previous articles relevant to early birds and avian origins see…
- Feathers and filaments of non-avian dinosaurs, part I
- Feathers and filaments of dinosaurs, part II
- Long and Schouten’s Feathered Dinosaurs, a review
- Tet Zoo picture of the day # 24 (on archaeopterygids)
- A stunning new Mesozoic bird… well, new-ish
- A quick history of tree-climbing dinosaurs
- Epidexipteryx: bizarre little strap-feathered maniraptoran
- A month in dinosaurs (and pterosaurs): 1, therizinosauroid fuzz
- A month in dinosaurs (and pterosaurs): 2, of alvarezsaurids and avialians
Refs – –
Bundle, M. W. & Dial, K. P. 2003. Mechanics of wing-assisted incline running (WAIR). The Journal of Experimental Biology 206, 4553-4564.
Dial, K. P. 2003a. Wing-assisted incline running and the evolution of flight. Science 299, 402-404.
– . 2003b. Evolution of avian locomotion: correlates of flight style, locomotor modules, nesting biology, body size development, and the origin of flapping flight. The Auk 120, 941-952.
– ., Jackson, B. E. & Segre, P. 2008. A fundamental avian wing-stroke provides a new perspective on the evolution of flight. Nature 451, 985-989.
Ellenberger, P. 1977. Quelques precisions sur l’anatomie et la place systematique tres speciale de Cosesaurus aviceps. Cuadernos Geología Ibérica 4, 169-188.
– . & de Villalta, J. F. 1974. Sur la présence d’un ancètre probable des oiseaux dans le Muschelkalk supérieur de Catalogne (Espagne). Note préliminaire. Acta Geológica Hispánica 9, 162-168.
James, F. C. & Pourtless, J. A. 2009. Cladistics and the origins of birds: a review and two new analyses. Ornithological Monographs 66, 1-78.
Olshevsky, G. 1991. A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Publications Requiring Research, San Diego.
– . 1994. The birds first? A theory to fit the facts. Omni 16 (9), 34-86.
Peters, D. 2000. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106, 293-336.
Renesto, S. 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106, 157-180.
– . & Binelli, G. 2006. Vallesaurus cenensis Wild, 1991, a drepanosaurid (Reptilia, Diapsida) from the Late Triassic of northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 112, 77-94.
Senter, P. 2004. Phylogeny of Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology 2, 257-268.
Turner, A. H., Pol, D., Clarke, J. A., Erickson, G. M. & Norell, M. A. 2007. A basal dromaeosaurid and size evolution preceding avian flight. Science 317, 1378-1381.