FANDOM


BCF FW

Longisquama,

The Birds Came First-hypothesis (abbreviated "BCF") is a non-standard, non-mainstream hypothesis proposed and developed by American paleontologist George Olshevsky in the early 1990s. The hypothesis, or ecomorphological and phylogenetic scenario, is a radical expansion of the possibility that some dinosaurs are secondarily flightless, arguing that all dinosaurs are "postvolant". It suggests that there is a close relationship between dinosaurs and birds, merely given this relationship, it is just as likely that dinosaurs descended from birds as the other way around. Shortly after reading Gregory S. Paul's Predatory Dinosaurs of the World, Olshevsky realized that the arguments expounded there for the secondarily flightlessness of the Maniraptora might well be adapted to argue for the same condition in all Theropoda, indeed in all Dinosauria.

The hypothesis does not propose that birds in the proper sense evolved earlier than did other dinosaurs or other archosaurs: rather, it posits that small, bird-like, arboreal archosaurs were the direct ancestors of all the archosaurs that came later on (proper birds included). Olshevsky was aware of this fact, and apparently considered the alternative acronym GOODD, meaning George Olshevsky On Dinosaur Descendants. This was, of course, meant as opposite to the BADD (Birds Are Dinosaur Descendants): the term George uses for the 'conventional' or 'mainstream' view of avian origins outlined in the first two paragraphs above. 'BADD' is bad, according to BCF, as it imagines that small size, feathers and arboreal habits all evolved very late in archosaur evolution, and exclusively within maniraptoran theropod dinosaurs.

"BCF" accepts the close relationship between dinosaurs and birds, but argues that, merely given this relationship, it is just as likely that dinosaurs descended from birds as the other way around. It should be emphasized that the term "birds" in this case refers to a morphological state, not to Aves as they have been cladistically defined. Olshevsky does not claim that the branch leading to modern birds split off from other dinosaurs very early; in fact precisely the opposite: he thinks that Aves is but the most derived expression of a vast diversification of flying dinosaurs all through the Jurassic.

BCF admits that most dinosaurs found are large and very derived in morphology compared to a hypothetical flying ancestor, and it also accepts the results of cladistic analysis connecting these large species into a cladogram, suggesting that the intermediate forms were also large. BCF could have avoided this problem by claiming that just a very basal form was volant and all subsequent forms large. Instead Olshevsky resorted to a far more radical position by emphasising the point that a cladogram doesn't logically imply the morphology of its intermediate steps. He claimed, basing himself partly on Cope's law, that there was a hidden stem lineage of small arboreal forms that during the Mesozoic was the real engine driving dinosaurian evolution, generating time and again larger ground-dwelling species. The smallness and arboreality of the forms was used to explain the fact that they rarely left a trace in the fossil record. Of course being small and arboreal doesn't imply the capacity to fly and Olshevsky allowed his hypothesis to diverge in three subhypotheses: the weakest was that the stem line consisted of small tree-living species; the stronger was that these could glide; the strongest that they possessed full powered flight.

Acceptance in paleontological circlesEdit

BCF has not found acceptance among professional paleontologists. It was only published twice, once in a magazine, Mesozoic Meanderings, which Olshevsky himself produced, and once in the popular-science magazine Omni.[1] Paul perfunctorily dismissed the hypothesis in his Dinosaurs of the Air; in the peer-reviewed literature it is never even mentioned as such. However this does not mean the hypothesis has been completely ignored by professionals. Olshevsky is a well-known figure among dinosaur enthusiasts in the USA and has been for many years a very active participant in the various internet fora dedicated to the study of dinosaurs. This has led to much debate about BCF. The main objections from the professional side are that the scenario as a whole is too vague to be testable and that the empirical support for the most interesting subhypothesis — full flight capacity — is poor. Only for the group of the Tetanurae, which are already quite derived theropods, are there some slight indications, and these can be explained as exaptations. Because of the many convergences needed, BCF is also not very parsimonious when analysed from a cladistic point of view, as it implies that flight was lost many times. The parsimony problem would only be remedied if many flying forms would be found basal to the various groups. Current paleontological consensus is that dinosaurs started as, and largely remained, ground-dwelling forms, that most major branching points in Mesozoic dinosaurian phylogeny were not volant and that it were only the members of the derived dinosaurian clade Maniraptora that took to the trees — and to the air — during the Jurassic.

Three problemsEdit

Olshevsky points specifically to three problems: the 'time problem', the 'size problem', and the 'wing problem'.[1][2]

'Time problem'Edit

The 'time problem' refers to the fact that basal birds (specifically, archaeopterygids) are older than other paravians. It also points to the fact that basal birds (Jurassic archaeopterygids) predate the flightless maniraptorans that are, according to the standard phylogeny, more basal than birds. Therefore, flightless maniraptorans (the mostly Cretaceous dromaeosaurids, oviraptorosaurs) are more likely derived from archaeopterygid-like ancestors, rather than vice versa.

The fossil record indicates, Olshevsky argues, that the large, flightless maniraptorans (dromaeosaurids and so on) are more likely to be the descendants of the little, flying, bird-like forms, not the other way round. This idea did not originate with Olshevsky: Greg Paul first proposed during the 1980s that dromaeosaurids and other bird-like coelurosaurs might be the secondarily flightless descendants of archaeopterygid-like ancestors.[3][4][5]

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, and a troodontid is now known from the Morrison Formation, for example. 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.

'Size problem'Edit

The second problem proposed by Olshevsky - the 'size problem' - asserts that the apparent miniaturisation of proto-birds required for the standard theory of avian origins is problematical given that animals tend to become larger during their evolution, not smaller (the infamous Cope's Rule). In the standard theory, no obvious explanation for the miniaturisation of the theropod dinosaurs ancestral to birds exists.[1][2] BCF gets round this (supposedly) by arguing that bird ancestors were small all along (going all the way back to the archosaurian common ancestor), and that the big archosaurs (ALL of them) evolved (on many, many separate occasions) from these small ancestors. In other words, BCF is compatible with Cope's Rule.

'Wing problem'Edit

Finally, BCF points to the 'wing problem'. The standard theory requires that the avian wing evolved gradually from a limb originally used for predation: Olshevsky (2001a) argued that no convincing model 'explaining' this transition had been presented (he noted that various different hypotheses have been suggested: that wings evolved as exaptations from feathered arms originally used as insects traps, for shading eggs, for providing thrust, for aiding manoeuvrability when running, and so on). BCF therefore follows the same logic as that applied by those ornithologists who favour a non-dinosaurian origin for birds: that wings 'cannot' have evolved in a terrestrial context, but must instead have evolved in an arboreal setting, in animals that used their forelimbs to slow or control their movement during leaping, parachuting, or gliding.

Solutions and evidenceEdit

In order to provide a solution that takes account of these 'three problems', BCF kind of combines the 'trees down' model of avian origins favoured by some ornithologists (and by most paleontologists in the post-Heilmann, pre-Ostrom era) with the secondary flightless hypothesis proposed by Greg Paul. BCF proposes that dinosaurs and other archosaurs started their history as small, quadrupedal tree-climbers, already kitted out with spiny 'proto-feathers', and that a 'central lineage' of small, tree-climbing forms connected these Permo-Triassic forms with the birds proper that evolved later on. All of the key innovations that led to the modern avian condition - skeletal pneumaticity, endothermy, feathers, wings, reduction and loss of the 4th and 5th fingers, reduction of lateral toes and modification of the hallux, reduction and loss of teeth, tail stiffening and so on - evolved in this single unbroken lineage, the hypothetical members of which were referred to by Olshevsky as 'dino-birds'. Dino-birds perhaps resembled the numerous small, arboreal theropods - the arbrosaurs - invented for Dougal Dixon's fictional work The New Dinosaurs. Whereas the conventional theory provides no obvious explanation for the incremental appearance of 'avian' characters in theropods and other archosaurs, BCF posits that they all evolved as logical improvements to the arboreal, gliding dino-bird lifestyle.

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. Scansoriopterygids, for example, 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 specializations.

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.[1] It's a weird climbing beast with a prehensile tail, described by Renesto[6] 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.

Longisquama and other theoriesEdit

Cosesaurus1

Cosesaurus, a small, quadrupedal reptile from the Triassic of Spain thought to be a dino-bird.

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.[7]Cosesaurus, a small, quadrupedal reptile from the Triassic of Spain - was also suggested by Olshevsky[8] 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 is now agreed that Cosesaurus is a protorosaur, and some paleontologists[9] interpret it as a close relative of pterosaurs, (with both taxa being nested within Protorosauria), and the impressions on the tail are apparently sedimentological artefacts.[10][11] 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.

Olshevsky's views on non-dinosaurian archosaurs aren't really that fringe given what is known about paleontology: he imagines crurotarsans formed a clade that diverged early on from the pterosaur-dinosaur clade, and the sorts of relationships that he's proposed for phytosaurs, rauisuchians, aetosaurs, crocodilians are so on aren't that different from standard phylogenies. However, among the most controversial of his proposals is regarding the enigmatic Triassic reptiles Megalancosaurus and Longisquama are dinosaurs, and part of a large group (termed Theropodomorpha) that also includes Marasuchus, Lagerpeton, herrerasaurids and theropods.[1][8]Saurischia is not monophyletic in Olshevsky's scheme, and sauropodomorphs and ornithischians are united in Phytodinosauria (an association originally proposed by Bakker, and later mooted by a few workers, but otherwise no longer maintained by anyone). Within Sauropodomorpha, sauropods are regarded as the most basal clade as, in contrast to other taxa, they possess large fifth toes.[12]

ReferencesEdit

  1. ^ a b c d e Olshevsky, G.. 1994. The birds first? A theory to fit the facts. Omni 16 (9), 34-86.
  2. ^ a b Olshevsky, G.. 2001a. The birds came first: a scenario for avian origins and early evolution, 1. Dino Press 4, 109-117.
  3. ^ Paul, G. S. 1984. The archosaurs: a phylogenetic study. In Reif, W.-E. & Westphal, F. (eds) Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Attempto Verlag (Tübingen), pp. 175-180.
  4. ^ Paul, G. S. 1988a. The small predatory dinosaurs of the mid-Mesozoic: the horned theropods of the Morrison and great Oolite - Ornitholestes and Proceratosaurus - and the sickle-claw theropods of the Cloverly, Djadokhta and JudithRiver - Deinonychus, Velociraptor and Saurornitholestes. Hunteria 2 (4), 1-9.
  5. ^ Paul, G. S. 2002. Dinosaurs of the Air: the Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press, Baltimore.
  6. ^ 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.
  7. ^ James, F. C. & Pourtless, J. A. 2009. Cladistics and the origins of birds: a review and two new analyses. Ornithological Monographs 66, 1-78.
  8. ^ a b Olshevsky, G. 1991. A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Publications Requiring Research, San Diego.
  9. ^ Peters, D. 2000. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106, 293-336.
  10. ^ Ellenberger, P. 1977. Quelques precisions sur l'anatomie et la place systematique tres speciale de Cosesaurus aviceps. Cuadernos Geología Ibérica 4, 169-188.
  11. ^ Ellenberger, P. & 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.
  12. ^ Olshevsky, G. 2001c. Dinosaurs 2001. Column 3: Isanosaurus. Dino Press 4, 92-95.
Community content is available under CC-BY-SA unless otherwise noted.