George Olshevsky (June 12 1946 - December 12, 2021) was a freelance editor, writer, publisher, paleontologist, and mathematician living in San Diego, California.
Olshevsky maintained the comprehensive online Dinosaur Genera List. He championed of the Birds Came First hypothesis in the descent of birds debate, which states that all dinosaurs are the descendants of small arboreal and perhaps flying ancestors.
Birds Came First[]
- Main article: Birds Came First
The Birds Came First-hypothesis ("BCF") is a radical expansion of the possibility that some dinosaurs are secondarily flightless, arguing that all dinosaurs are "postvolant". The hypothesis, or ecomorphological and phylogenetic scenario, was developed by Olshevsky in the early nineties. 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.
"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.
BCF did 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. 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.
References[]
- ^ Olshevsky, G. (1994). "The birds first? A theory to fit the facts - evolution of reptiles into birds". Omni, June, 1994