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Re: Sapeornis



ekaterina A wrote-

> The tree shown in the Zhou et al shows Sapeornis as
> the next most primitive bird after Archaeopteryx.
> However, in the text they talk of the long ulna and
> fused Metacarpal 2 and 3. Al these going through
> Paul's book seem to be derivations of more advanced
> birds. Also it seems to have a nice stout pygostyle.
> So is Sapeornis really so primitive? Have any of you
> all (Messrs. Mortimer or Headden?) added Rahonavis and
> seen what it does to the tree? I have not read this
> Jehlornis paper,but will be reading it shortly. Is
> Sapeornis truely the largest avian dinosaur of the
> Early Cretaceous?

Sapeornis has an ulna subequal to the humerus in length (104%).  Basal birds
generally have lower ratios (Confuciusornis 82-90%, Archaeopteryx 86-90%,
Changchengornis 95%, Longipteryx 98%, Jeholornis 99%) while enantiornithines
range from 100% in Sinornis to 115% in Longchengornis.  However, the basal
Jibeinia and Eocathayornis have high ratios of 103% and 111% respectively.
So this is not unexpected for Sapeornis.
Confuciusornis also has a semilunate carpal fused to its second and third
metacarpals, but not the first.  So this is not necessarily a very derived
character.  Manual fusion is variable in basal birds, ranging from
completely unfused to as fused as neornithines.  Both ontogenetic and
individual variation may partially account for this.
The pygostyle of Sapeornis consists of about four vertebrae and is
comparable to basal euornithines in size.  The elongate pygostyle of
confuciusornithids and enantiornithines is probably a pygostylian
synapomorphy reversed in euornithines.  One thing I'm irritated with is the
assumption the elongate pygostyle was the primitive condition for all
pygostyled birds.  Notice in Jeholornis' data matrix, Jeholornis is coded as
having a "long pygostyle made of more than four vertebrae"?  Just because
that's the primitive state in their character list.  It's ridiculous.
Sapeornis is quite primitive in its sutured scapulacoracoid, short coracoid,
manual phalanx II-2 longer than II-1, two phalanges on digit III, fibula
that reaches the tarsus and free distal tarsal.
Rahonavis and Sapeornis may be very close phylogenetically.  These basal
birds have very odd characters illustrating the need for a detailed
phylogenetic analysis, which I'm in the process of performing.  Sapeornis
differs from Rahonavis in lacking dorsal pleurocoels, having a sutured
scapulacoracoid, distally tapered scapula, no mid-dorsal ischial process, no
obturator process, a shorter pubic symphysis, no pubic foot, tibiotarsus, a
fibula that reaches the tarsus, and a tarsometatarsus.  Those are the most
obvious differences, of which most are more derived than Rahonavis.
Jeholornis was probably longer than Sapeornis simply because of its tail,
though Sapeornis was more massive.  These two taxa were indeed larger than
any other known Early Cretaceous avians.

Mickey Mortimer