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David Marjanovic
wrote-
>What is a
metataxon?
A taxon people are too lazy to find apomorphies of,
or else a nomen dubium they really don't want to drop. ;-) Okay,
seriously, it's a taxon with no apomorphies, but which can be distinguished from
other taxa of the same time period. I strongly disagree with this concept,
as I feel people think of apomorphies only as these obvious new characters and
not as proportional or shape differences, which are just as valid in my
opinion. The only instance I can see where a metataxon might be
appropriate is if some elements were named that were absolutely identical in
morphology to another species, but this "metataxon" was over thirty million
years older/younger than the other species and preferrably on another
continent (though even that has the paleogeography thing to cancel it
out). Some people think ancestors of species are metataxa and that some
fossil specimens are actually ancestors. This may work well for well
sampled groups like sharks or mammals where we can see one form grading into
another through time, but the dinosaurian record is far less complete. Any
hypothetical ancestor is going to be different from the main stock it's evolving
from, as it is developing mutations that will make it a new species. If
these differences cannot be seen in the fossil (which is very possible, as so
little anatomy is preserved), it should be classified as a member of whatever it
evolved from. This is obvious and probably done quite often, although we
would never know. If the differences can be seen in the fossil, then use
the differences to define it and make it a new species. Whatever it
eventually evolved into can be defined as another species as it has yet more
characters that developed after its ancestor. This is all hypothetical and
probably useless at the present, as dinosaur taxonomy and diversity is too
poorly known to be worrying about species concepts yet.
>How is Neotheropoda defined at the
moment?
All descendents of the most recent common ancestor
of Ceratosaurus and Neornithes (Padian et al., 1999). Hmm. I
suppose coelophysoids and Dilophosaurus wouldn't be neotheropods then.
Time for another name.... :-)
>>Seven additional
synapomorphies were found (premaxillary body in front of external nares longer
than body below the nares, and angle between anterior margin of the premaxilla
and alveolar margin less than 70 degrees; presence of a constriction between the
articulated premaxillaries and maxillaries;
>Are these three related
to the subnarial gap?
No. The first is found in Torvosaurus,
Neovenator, Compsognathus, Erlikosaurus, Archaeopteryx, etc.. The angle of
the anterior margin is actually a second part of that same character. The
second is also found in oviraptorids, which lack the subnarial gap.
>I've often seen "no
fanglike teeth in the dentary" used as an apomorphy of Tetanurae, implying that
their presence is plesiomorphic. Where does this attitude come
from?
Another one of those "assumed" characters like the
fibular sulcus. Note that neither Euparkeria, ornithischians,
sauropodomorphs, Eoraptor, Herrerasaurus or abelisaurids have enlarged anterior
dentary teeth. Spinosaurids and both species of Magnosaurus do
though.
>>ascending process
of the maxillary offset from the anterior rim of the maxillary body and anterior
projection of the maxillary body longer than high;
>Related to the typical
skull shape of most large theropods?
No, as Ceratosaurus, abelisaurids and
tyrannosaurids lack it.
>>cervical vertebrae
strongly opisthocoelous;
>Would this add rigidity
or flexibility?
Flexibility. Opisthocoelous centra have a
hemispherical front end to articulate with a large socket in the back of the
proceeding vertebra.
>>metacarpal I very
stout and approximately as broad as long.
>Sounds like a
plesiomorphy, AFAIK...
Allosaurus' width/length ratio is 72, Torvosaurus'
and Poekilopleuron's are 70, Acrocanthosaurus' is 72. The supposedly
non-carnosaurian "Szechuanosaurus" zigongensis' is 58, Xuanhanosaurus' is 60,
Ceratosaurus' is 36 and Dilophosaurus' is 39. If identified correctly,
Carnotaurus' is 76, but it doesn't look like a metacarpal to me.
"Szechuanosaurus campi's" first metacarpal isn't described well enough to
determine its robustness. So, that character seems valid if specified to
be "metacarpal I more than 65% as wide as long". The only other theropod I
know of with such a wide first metacarpal is Therizinosaurus (67%).
> Interesting... is Magnosaurus no
longer considered a nomen dubium (in which case the much more beautiful name
Eustreptospondylus could remain...)?
It's a good example of what I've been saying about
the recent trend to make all poorly known taxa nomina dubia. Magnosaurus
nethercombensis can be distinguished from other large theropods, so is a valid
taxon. Unless you think the minor differences between M. nethercombensis
and M. oxoniensis are enough to separate the two into different genera, it looks
like Eustreptospondylus will be a junior synonym.
>This sounds a lot like PDW (where
Yangchuanosaurus was lumped into
Metriacanthosaurus).
Indeed. And if they do turn out to belong to
the same family, Metriacanthosauridae (Paul, 1988) will have precedence over
Sinraptoridae (Currie and Zhou, 1993).
>I'm already waiting that Dryptosaurus
becomes a ceratosaur... ~:-|
Quite unlikely. Dryptosaurus has an enlarged
posterolateral surangular foramen, anteriorly concave pubic shaft, prominent
proximolateral ischial scar, upturned femoral head, wing-like lesser
trochantor, very tall astragalus without a transverse groove across the condyles
and a deeply concave anterior astragalar margin in ventral view. These are
all characters seen in coelurosaurs and especially tyrannosauroids.
I'm not sure where the idea that Deltadromeus is
related to Dryptosaurus originated. The two are both basal coelurosaurs
(my preliminary opinion on Deltadromeus until I compare it more closely to more
basal theropods) but are not easily comparable. Both have
somewhat similar deltopectoral crests, but Dryptosaurus' humerus is more
developed proximoposteriorly and seems more sigmoid. Deltadromeus has an
oddly anteroposteriorly narrow femoral head and an anterior trochantor that
starts further distally. Deltadromeus' lateral tibial condyle has an odd
posterior process, the cnemial crest is narrower and the incisura tibialis is
more excavated in proximal view. Deltadromeus has a better developed
proximomedial fibular fossa, but it is not as extensive
proximoposteriorly. Dryptosaurus' ascending process is much higher and
more pointed and it lacks the plesiomorphic transverse groove across the
astragalar condyles found in Deltadromeus. Metatarsal IV is much narrower
in Deltadromeus and it's proximal end is less triangular than Dryptosaurus and
tyrannosaurids, and lacks the notch found in those taxa. Deltadromeus'
humerus is 6% longer compared to femoral length, but its tibia is 6%
shorter. As you can see, the taxa are rather different, although a better
description of Deltadromeus would make it easier to compare them. I have
not found any synapomorphies that might unite the two and in astragalar
morphology at least, Dryptosaurus is more derived.
>>Rauhut assigned
Bahariasaurus to the Carcharodontosauridae in 1995 based on characters of
referred material. The holotype is said to lack carcharodontosaurid
synapomorphies and be very close (if not identical) to
Deltadromeus.
>So it's back in incertae sedis?
Bahariasaurus is even more difficult to compare to
Deltadromeus than Dryptosaurus was. Bahariasaurus is based on dorsals,
sacrals, a pubis and proximal ischium. As the pubis of Deltadromeus is
probably an ischium, dorsal vertebrae of the holotype are fragmentary and not
illustrated, and sacral vertebrae are unknown, only the proximal ischia can be
compared. Sereno distinguished Deltadromeus from Bahariasaurus by two
pubic characters (now useless, as no pubis is known for Deltadromeus' holotype)
and the ilial peduncle of the ischium being narrower. Deltadromeus'
ischium certainly needs to be illustrated better, but I don't think the ischial
character sounds very convincing. There is no reason to assume that
Bahariasaurus is a nomen dubium, as it has yet to be compared in depth to other
theropods. I think Bahariasaurus is probably a senior synonym of
Deltadromeus, but will wait until I do a more detailed study to make any
conclusions.
>>The absence of
transverse processes in Compsognathus is no longer certain.
>Sinosauropteryx has, IIRC, lots of them.
Possibly. Chen et al. (1998) state, "There
are at least four pairs of caudal ribs in NIGP 127586, and more distal caudals
have low bumps in this region that could also be interpreted as transverse
processes." So there are 4+, until Currie describes it better.
Rauhut saw the specimen and has it coded as possessing more than 16 caudals with
transverse processes, so that may be telling.
Mickey
Mortimer |