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

Re: segnosaurs not therizinoid??



Ken Kinman wrote-

>      I was just reading an online article by Jack Conrad (see link below),
> indicating his belief that segnosaurs are probably not closely related
> Alxasaurus, and that combining them into a clade was premature and
probably
> erroneous.
>      Has anything really changed (new fossil material) in the last two
years
> that would challenge his view?  Do others share the view that
> therizinosauroids (incl. segnosaurs) are polyphyletic?

Although I must congratulate Jack for his professional presentation, there
are several flaws in his reasoning.  It must be said that he may not still
have this position, as the article was written in May 1999 before the
description of Beipiaosaurus.
First, by only commenting on "indisputable" remains of segnosaurs, he can
easily bypass the referred hindlimb of Therizinosaurus (GI 100/45).
Although I am currently in the process of getting the description of this
translated, the illustrated pes and tarsus are clearly segnosaurian.
Moreover, it was discovered with an incomplete forelimb and manual ungual,
no doubt ensuring its proper referral to Therizinosaurus.  Segnosaur
synapomorphies include the ascending process curving medially and the
unreduced first metatarsal.  This specimen clearly indicates Therizinosaurus
is related to Segnosaurus and Erlikosaurus, even if one ignores the forelimb
characters (ligament pits absent in manual phalanges, weakly developed
manual ungual flexor tubercles, etc.).  If anyone wants a scan of this
specimen, contact me offlist.
It might be interesting to note Nanshiungosaurus does have dorsal
pleurocoels, contra Conrad (Makovicky, 1995).
The many differences cited between Alxasaurus and Therizinosaurus and
"segnosaurids" mean nothing phylogenetically unless one finds they are
synapomorphically shared with another group.  Conrad suggests
Therizinosaurus may be cloely related to ornithomimosaurs based on
similarities to Harpymimus, but this taxon is receiving a redescription that
will substantially alter our opinion of it, so does not help us assign
Therizinosaurus to another group.  The characters suggested are either
valid, although also present in other taxa (humeral ends expanded in same
plane- also in alvarezsaurids and ornithurines) or symplesiomorphic (similar
metacarpal proportions).  Conrad compares Alxasaurus favorably with
oviraptorosaurs, while suggesting "segnosaurids" aren't theropods.  As the
current consensus is that segnosaurs are the sister group of
oviraptorosaurs, it wouldn't be surprising to see that a basal form like
Alxasaurus would resemble oviraptorosaurs more.  Alxasaurus actually shares
many synapomorphies with therizinosaurids-
- cervical neural arches elevated
- cervical zygapophyses widely spaced
- zygapophyseal extremities defining a shape intermediate between a
right-angled X and a laterally compressed X in dorsal outline
- flexor groove between ulnar and radial condyles deep and narrow
- short expanded preacetabular process
- lateral tuberosity on distal postacetabular process
- tibia shorter than femur
- shortened metatarsus (<30% of femoral length)
- metatarsal I articulates with tarsus
- pedal unguals very narrow (<17% of length)
Similarily, there are a whole lot of synapomorphies shared between
segnosaurs and oviraptorosaurs showing they are theropod.  As I posted
previously on the vrtpaleo list-

The following have all been said to be diagnostic of a
therizinosaur-oviraptorosaur group.  Many of them are seen in other
coelurosaurs as well, which further suggests many are not actually
synapomorphic of a segnosaur-oviraptorosaur group.  I support those with an
asterisk next to them.
*- posterior section of premaxilla toothless (modified from Makovicky and
Sues, 1998)
The original character referred to a completely toothless premaxilla
(Erlikosaurus, oviraptorids), but the basal oviraptorosaur Caudipteryx has
anterior premaxillary teeth.
- rim of antorbital fossa well developed (Xu et al., 1999)
This is present in Erlikosaurus and Chirostenotes, but the condition in
oviraptorids is unclear.  Caudipteryx seems to lack it.
- maxillary fenestra absent (Makovicky and Sues, 1998)
At least some oviraptorids have maxillary fenestrae, as does Caudipteryx,
although Erlikosaurus, Chirostenotes and the holotype of Oviraptor
philoceratops lack them.
*- promaxillary fenestra absent
Erlikosaurus, Chirostenotes and O. philoceratops lack one, Caudipteryx has a
promaxillary fossa and oviraptorids are variable.  The crestless oviraptorid
ZPAL MgD-I/95 has four maxillary foramina/fenestrae, two inside the
antorbital fossa and two outside.  The Oviraptor philoceratops holotype has
no maxillary fenestrae inside its antorbital fossa.  GI 100/42, referred to
O. philoceratops, has a large maxillary fenestra and one or perhaps two
small fossae outside the antorbital fossa.  Conchoraptor seem to have no
antorbital fenestra at all, just an antorbital fossa.
*- nasal subequal in length to frontal
True in Erlikosaurus, Caudipteryx and oviraptorids.
- vomers extend posteriorly to basicranium (Russell and Dong, 1994)
This is true of Erlikosaurus, but not oviraptorids.
*- ectopterygoid lateral to palatine
True in Erlikosaurus and oviraptorids.
- no fenestra between ectopterygoid and palatine (Russell and Dong, 1994)
Not true in Erlikosaurus, oviraptorids or (probably) Chirostenotes.
- reduced basipterygoid processes (Russell and Dong, 1994)
True in Erlikosaurus, Chirostenotes and oviraptorids.
- lateral depression in middle ear region of braincase (Makovicky and Sues,
1998)
Present in Erlikosaurus and Chirostenotes, but not in oviraptorids.
*- dentary symphysis decurved
True in segnosaurs, Caudipteryx, Microvenator, Chirostenotes and
oviraptorids.
*- dentary symphysis deflected medially (Makovicky and Sues, 1998)
Same as above.
- coronoid absent (Russell and Dong, 1994)
Most oviraptorids don't preserve coronoids, so they were thought to be
lacking in the group until a new Mongolkiamn specimen was found with a tiny
one.  The supposed absence in Erlikosaurus, Segnosaurus and Chirostenotes
must be reexamined.  A tiny bone in the naris of a Caudipteryx specimen may
be a coronoid.
*- two pairs of cervical pleurocoels (Makovicky and Sues, 1998)
True in Microvenator, Chirostenotes, oviraptorids, the El Brete
caenagnathoid and a Kazakhstan therizinosauroid.  Only one pleurocoel is
preserved in Alxasaurus.  Nomingia lacks this trait.
- peduncular foramina on cervicals (Frankfurt and Chiappe, 1999)
Present in the Quarry 9 segnosaur-oviraptorosaur, therizinosauroids,
Chirostenotes and the El Brete caenagnathoid.
*- ventral sulcus on cervical centra flanked by ventrolaterally directed
ridges (Frankfurt and Chiappe, 1999)
Present in the Quarry 9 segnosaur-oviraptorosaur, therizinosauroids,
Chirostenotes and the El Brete caenagnathoid.
*- cervical prezygopophyses separated by a U-shaped space (Frankfurt and
Chiappe, 1999)
Present in therizinosauroids, Microvenator, oviraptorids and the El Brete
caenagnathoid.
- all dorsal vertebrae pleurocoelous (Makovicky and Sues, 1998)
Present in Beipiaosaurus, "Nanshiungosaurus" bohlini, Nomingia, Microvenator
and oviraptorids.  Alxasaurus and Nanshiungosaurus brevispinus lack dorsal
pleurocoels, while Caudipteryx lacks them at least posteriorly.
*- caudal vertebrae decrease in length posteriorly (Makovicky and Sues,
1998)
True for Alxasaurus, Caudipteryx, Nomingia, Microvenator, oviraptorids and
probably Chirostenotes.
- ulnar facet of humerus expanded, merges with entepicondyle (Russell and
Dong, 1994)
This seems to be absent in oviraptorids, but I haven't examined it in depth
yet.
- proximodorsal lip on manual unguals (Makovicky and Sues, 1998)
This is a highly variable character in both morphology and which unguals it
forms on.  Caudipteryx lacks it on unguals I and II (digit III lacks an
ungual).  Microvenator has it in the single ungual preserved.  Caenagnathids
show it on all unguals.  Most oviraptorids have it on some unguals
(Conchoraptor- I and II; GI 100/42- I, II and III; Oviraptor philoceratops-
not on II at least; Ingenia- none).  Alxasaurus shows it on II, but not I.
Therizinosaurus on I and II, but not III.  Beipiaosaurus on I, but not II.
- preacetabular process greatly expanded dorsoventrally (Russell and Dong,
1994)
True in segnosaurs, Chirostenotes and "Rinchania".  I don't feel the slight
condition in Caudipteryx, Microvenator and Nomingia is that different from
other theropods.  Ingenia and an undescribed crestless form certainly lack
it.
- postacetabular process tapered (Russell and Dong, 1994)
True in therizinosauroids, but not Beipiaosaurus, Caudipteryx, Nomingia,
Chirostenotes or oviraptorids.
*- pubic foot expanded anteriorly (Makovicky and Sues, 1998)
True in Segnosaurus, Enigmosaurus, Nanshiungosaurus, Caudipteryx, Nomingia,
Microvenator, Chirostenotes and oviraptorids.

Finally, new finds such as Beipiaosaurus and "Notheonychus" support
segnosaurian monophyly and theropod ancestry as well.

Mickey Mortimer