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Been out of town for a few days and have 1032 messages sent to me.Took
two days to read them. I read some interesting debates on therizinosaurs
and want to give you my two cents.
Therizinosaurs lack the basic prosauropod features. Prosauropods are
very differen osteologically. Prosauropods have a list of features that
unite them ( Galton; Dinosauria).
1) Skull about half the lengh of femur. This feature is seen in most
dinosaurs and seem to be a plastic feature.
2) Jaw articulation situatated slightly below the level of the
maxillary row. Seen in most herbivorous dinosaurs. It is not seen in
3) Dendition of small, homodont or slightly heterodont,
spatulate-shaped teeth with coarse,obliquely angled serrations. Compare
the pictures in the Dinosauria, Prosauropod and Segnosauria sections.
Therizinosaurs lack coarse, obliquely angled serrations.
4) Digit I of manus bears a twisted first phalanx and an enormous,
trechant ungual that is medially directed when hyperextended. Part of
the problem in the prosauropod/therizinosaur hypothesis is the
completely different strucures of the manus. The therizinosaur manus
lacks a twisted first phalanx. The therizinosaur manus bears three
manual digits of elongated, slender phalanxes, and it bears elongate,
laterally compressed unguals. The prosauropod manus is robust , bears
robust phalanxes, and robust >ventrally< compressed claws.
5) Digits II and III subequal in length and bear small, only slightly
recurved ungual phalanges. See above.
6) Digits IV and V reduced and lack ungual phalanges. This is not
useful because therizinosaurs lack digits IV and V.
7) Typical phalangeal formula 2-3-4 ( 3 or 4). Plesiomorphic.
8) Bladelike distal pubes form broad apron. This is the reverse of that
9) Pes with vestigal digit V. This is not useful because it is
homoplastic among several dinosaur groups.
As you can see the automorphies of the Prosauropoda are basically
absent in the Therizinosauridea.
I advocate that theriziunosaurs are the sister group to the
Ornithomimosauria. Here are the synapomorphies.
1) Bulbous, pnematized parashenoid.
2) Teeth with a constriction between the root and crown.
3) Large acromion process on a broad, rectangular coracoid with a long
posteroventral process and a tubercle for the M.biceps.
4) Rectangular anterior protuberance on the scapula.
5) Elongate, strap-like posterior scapula end that is elongate
6) Long, slender humerus with elongate proximal deltapectoral crest (
7) Manual phalanges subequal ion length.
8) Reduction of the number of caudals ( 25-30.)
If this is right therizinosaurs are therefore Maniraptoriformes,
Arcometatarsalians, and Bullatosaurians.
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