POSITION OF ARCHAEOPTERYX

["Matthew Troutman" <m_troutman@hotmail.com>]

<<I happened to pick up a copy of Feduccia`s book  "The Origen and 
Evolution of Birds", and, though I don`t quite agree on  his position, 
find that the book is filled with some interesting facts! Perhaps to 
clarify some of the above statements, here are some quotes...(preface 
viii)   "The Mezozoic era,....was a period of adaptive radiation.... 
Predominant in this radiation were the recently discovered "opposite", 
or enantiornithine, birds, so named because the three metatarsal bones 
fused from proximal to distal, the opposite direction of that of modern, 
ornithurine birds." "These opposite birds are included in the same 
subclass, the Sauriurae, with the urvogel, Archaeopteryx....">>

Look up the refs within the book, they are filled with much more facts.  
Anyway, the statement above is a recap of the major arguments of Larry 
Martin, who believes that _Archaeopteryx_ is not related to modern 
birds, but related to enantiornithines.  There WAS a vast radiation of 
enantiornithines in the Cretaceous, and their metatarsals were fused in 
a proximo-distal pattern like some specimens of _Archaeopteryx_.  The 
group of Archaeornithes + Enantiornithes is named Sauriurae by Larry 
Martin, who used this usage in his 1985 Archaeopteryx Paper, still one 
of the most cogent arguments for his position.  

<<So, I assume that the original statement,.. that the 
arctometatarsalian condition is " reversed " in archaeopteryx, and 
enantiornithine (sp),.. dosen`t imply that it is in the process of being 
undone, but that the condition, unlike the arctometatarsalian, has the 
bones fused principally at the proximal end. The Ornithurae would, as 
implied by Feduccia, have the arctometatarsalian condition, and 
therefore shouldn`t be grouped with the other two.>>

Feduccia nevers implies that!  There is no constriction of the 
metatarsals in ornithurines.  Their metatarsus fuses in a disto-proximal 
fashion, opposite that of the enantiornithines and _Archaeopteryx_.  
Other pecularities include a tarsal cap that covers the metatarsus in a 
sheet-like fashion and a hypotarsus with "ligamental canals".

I don't think that Feduccia has ever acknowledged that theropods are 
bird-like except in regards of hindlimb structure.  And he certainly did 
not think that ornithurines have the arctometatarsalian condition.  He 
did not even site Holtz's 1994 paper.  

<<As to what it all means to evolutionary theory in general, I think 
Feduccia is implying that archie is not to be considered as a direct 
link to the modern ornithurine birds.      Larry Febo (dinosaur 
researcher).>>

This was first outlined by Whetstone (I must ask, what happened to him 
after 1983?) 1983 in his infamous paper on the London braincase of 
_Archaeopteryx_.  He concluded that the squamosal in _Archaeopteryx_ was 
repositioned (not reduced or lost as sited by Martin), that the quadrate 
head only had a medial structure (modern birds have a double-headed 
quadrate that has two condyles; lateral, contacting with the squamosal, 
and medial, contacting the paraoccipital process and prootic), and there 
was a quadrate facet on the braincase on the "otoccipital" (presumed to 
be present in _Archaeopteryx_ by Whetstone because it would be similiar 
to crocodilians in this respect; fused exoccipital/opisthotic).  These 
interpretations made it similiar to the enantiornithines (which do not 
have these structures as most recently found), along with details in the 
shoulder girdle and tarsometatarsus.  Martin (1983; Current Ornithology 
1) endorsed these interpretations.  His most recent strong advocacy was 
in 1991 (Origins of the Higher Groups of Tetrapods) where he discounted 
the interpretations and criticisms of Alick Walker (1985; Archaeopteryx 
Conference) and gave a detailed (and mostly accurate interpretation of 
the skull.

As mentioned above, Walker (1985) critisized Whetstone's interpretations 
and offered a detailed revision of the London braincase (an analysis 
that does not differ much from Whetstone's except in some details).  
Within, Walker concluded that the 'facies articularis pro quadrato' of 
Whetstone was really the smooth threshold to the posterior tympanic 
recess.  Walker acknowledged that there are little signs of a squamosal 
articulation on the London braincase and turned to the Eichstaat skull 
for the squamosal.  He concluded that the quadrate head fragment of 
Whetstone was really a small squamosal crushed medially.  Martin (1991) 
disagreed, arguing that it is the threshold to the posterior tympanic 
recess and was a medial strucuture crushed laterally.  Whetstone argued 
(rightly) that the squamosal could not cover the periotic sinus 
(superior tympanic recess) so it must have been repositioned.  Walker 
concurred to a point, but argued that the _Archaeopteryx_ specimens were 
juveniles and the squamosal grew throughout ontogeny and covered the 
supperior tympanic recess.  The process is too complex and lengthy to 
discuss in full here, but to summarize, he concluded that the braincase 
enlarged with the squamosal and various bones (such as the postorbital) 
were lost in adulthood and the enlargement could have caused incipent 
prootic and opisthotic contacts with the quadrate foreshadowing the 
condition in birds.  The ontogeny of the modern avian skull supports 
this interpretation.  

In 1996 (JVP 16), Elzanowski and Wellnhofer offered interpretation of 
the best preserved of all _Archaeopteryx_ skulls.  Within, they 
described a quadrate that articulated mainly with the squamosal, as 
Walker conclude, and a squamosal that possibly could not cover the 
superior tympanic recess as Whetstone interpreted.  The skull, as 
interpreted, probably had a postorbital, and was simultaneously 
theropod-like and extremely bird-like.  The skull supports details of 
both Whetstone's and Walker's interpretations.  I am beginning to think 
that Walker's braincase enlargement theory was probably correct to a 
point, but that the squamosal did not expand much and was rather small.  

Anyway, what does the skull of _Archaeopteryx_ tell us about its 
relationships to modern birds?  It, for the moment, supports that it 
could be ancestral to modern birds and was not basal only to 
enantiornithines.  

Matt Troutman

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