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

Re: titanosaurs



In a message dated 12/2/02 11:47:17 AM EST, qilongia@yahoo.com writes:

<< Many titanosaur features are found in it, in fact: non-forking haemal 
spines, neural arches of caudal vertebrae set forward on the centrum, neural 
arches of dorsal vertebrae short in height and the spine comprises less than 
1/2 the height of the vertebrae, as well it is caudally inclined, six sacral 
vertebrae, pubis longer than ischium, femur with distinct lateral tubercle 
and femur with deeper fibular condyle than tibial, resulting in a wide-gauge 
stance, manus without much articular area for extra phalanges, and metacarpal 
arch bound into a tight C-shape, ulna with distinct olecranon jutting far out 
from the humeral posterior margin in side view, and with a distinct medial 
crista separated from the proximal margin by a cleft or fossa, ulna 
distinctly shorter than the humerus by less than 75%, a squared off distal 
scapula and a gross scapulocoracoid shape identical to some titanosaurs. >>

This is what I call a "synapomorphy war": a deluge of incredibly minor 
characters of dubious taxonomic utility thrown together in a torrent 
purporting to show that a number of specimens belong to the same taxon. In 
truth, for any two specimens, there is at most >one< character that is a true 
synapomorphy for them (and it might not even be in the list of matching 
characters, such as the above). All the other similar characters are either 
convergences (because they arose independently in both lineages after they 
split from the common ancestor) or plesiomorphies (because they were already 
present in the lineage leading up to the common ancestor). The likelihood of 
two or more of these characters arising together at >exactly< the same time 
in the common ancestor species is nil. So--which of these characters, if any, 
is the unique synapomorphy that places Opisthocoelicaudia into a titanosaur 
taxon? How would you go about demonstrating this? (I already noted, for 
example, that non-forking haemal spines is a plesiomorphy for Dinosauria and 
should not even appear in this particular synapomorphy war.)