[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

*To*: <dinosaur@usc.edu>*Subject*: Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE*From*: "John V Jackson" <jjackson@interalpha.co.uk>*Date*: Mon, 5 Oct 1998 21:29:12 +0100*Reply-to*: jjackson@interalpha.co.uk*Sender*: owner-dinosaur@usc.edu

Matt Troutman: 4/10/98: >Another weakness of your hypothesis is its interpretation of secondary flightlessness in maniraptoriforms. A far better interpretation is that they are avian ancestors not avian descendents. < Just for a moment considering the chronological order of appearance in the fossil record (I?m referring to the non-pre-Archaeopteryx appearance of maniraptorans), how many paradoxical instances make a piece of evidence? Jurassic fossils are rarer than Cretaceous fossils; it is also hypothetically possible that if we assume maniraptorans gave rise to birds, it may have been early in maniraptoran existence (though the more this is true, the weaker the case ? see below). (By chance, only a small fraction of the evidence appears between Archaeopteryx and the end of the J (assuming the China stuff is K) so the principle needs only slight re-phrasing to refer to either ?Pre-Archae? or ?Pre-K?). Say the ratio of maniraptorans living in the K to those living in the J was A:B; Say the ratio of the chances of an indiv living in the K ever being found to that of one in the J is C:D; The ratio of the chances of a maniraptoran we have found coming from the K to from the J is (A/B)*(C/D). The probability of any maniraptoran we have found coming from the K is 1/(1 + 1/ ((A/B) * (C/D)) ). The chance of N maniraptoran fossils all appearing after Archaeopteryx is this expression raised to the power of N. It is interesting to plug various values into this: A/B=3.01; C/D=2; N=30; gives a probability of 0.01 . Very unlikely, and bad news for BADD. However, perhaps a more friendly set of inputs would be if K mani?s were 100 times as numerous as J ones, and the the K was 5 times as fossiliferous as the J: A/B=100; C/D=5; N=30; gives a probability of 0.941 . Fine for BADD ? sort of. However this combination assumes maniraptorans hardly existed pre-Archaeopteryx!! And maybe N should be much bigger than 30. Perhaps the hypothetical pre-Archae maniraptorans were difficult to find because they were very small, and/or lived in conditions unhelpful for fossilisation ? they were living in forest trees for example. . . !!! Whatever assumptions we make, a BCF-type explanation is the only answer, using these probability calculations. Maniraptorans therefore did not give rise to birds since they all postdate the first bird ? unless they were effectively early birds already. To return to the original quote, a far better interpretation is that maniraptorans *are* avian descendents (though not necessarily pterosaur descendents). There have been claims of pre Archae. Jurassic maniraptorans, largely in the form of teeth, often unusually small. You may call them maniraptoran but they will probably be small aerial forms, or larger non-arborial descendents of them. JJ

- Prev by Date:
**Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE** - Next by Date:
**Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE** - Previous by thread:
**Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE** - Next by thread:
**Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE** - Indexes: