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Sorry not to have written a details segment in a while. Here's a theropod
that's been claimed to be part ornithopod or perhaps a Middle Jurassic
coelurosaur. It's neither, as we'll see below.
Chuandongocoelurus He 1984
C. primitivus He 1984
Etymology- "primitive hollow-tail from Chuandong",
after Chuandong, where the remains were found.
Bathonian-Callovian, Middle Jurassic Xiashaximiao Formation, Sichuan, China Holotype- (CCG 20010, subadult) (2.4 m, 12 kg) third cervical vertebra (61 mm), tenth cervical vertebra (69 mm), third dorsal centrum (65 mm), fourth dorsal vertebra (58 mm), proximal caudal vertebra (60 mm), partial scapula (282 mm), partial ilium, proximal pubis, proximal ischium, femur (201 mm), tibia (231 mm), astragalus, calcaneum, incomplete metatarsal II, phalanx II-1 (30 mm), metatarsal III (122 mm), metatarsal IV (114 mm), phalanx IV-1 (16.5 mm), phalanx IV-2 (9 mm), phalanx IV-3 (10 mm), phalanx IV-4 (12 mm), pedal ungual IV (18 mm) Diagnosis- lateral depression of third cervical
extends over 60% of central length; small anterodorsally projecting process
anterior to proximal caudal neural spines (also in some carnosaurs); scapula
less than 5.5 times longer than wide; base of preacetabular process less than
half acetabular height; prominent anteriorly projecting process lateral to
anterior trochantor on femur; medial condyle much larger than lateral condyle on
tibia.
Derscription-
Although described by He Xinlu back in 1984,
Chuandongocoelurus has been largely ignored in the literature. Comparing
the femoral length to Elaphrosaurus, it can be estimated to have measured 2.4
meters in length. Scaling from Paul's (1988) estimated mass of
Elaphrosaurus results in a weight of 12 kilograms. The unfused
neurocentral sutures in the dorsal vertebrae show this was a subadult, so this
was not its maximum size.
Of the two cervical vertebrae preserved, one is
clearly from a very anterior position due to it's slender and short
centrum. Glut (1997) refers to an axis, but the strong parapophyses
suggest it was a third cervical instead, as Carnotaurus and Dilophosaurus have
very reduced axial parapophyses (the latter has none). The centrum is
perhaps flat anteriorly and slightly convex posteriorly, with an anterior face
38% wider than tall. There is a deep lateral depression (pleurocoel?)
extending from the posterior border of the centrum to beneath the
diapophyses. The diapophyses extend ventrolaterally to almost contact the
parapophyses. The prezygopophyses are broken off, but thir bases show they
were massive. The posyzygopophyses are also broken and were more
slender. There is a large postzygopophyseal-central choana that has a step
on the ventral edge. The neural spine is not well preserved, but was low
and rounded.
The other cervical is from the posterior portion of
the series as seen by the postzygopophyses extending past the centrum and the
craniocaudally short neural spine. Comparison with Elaphrosaurus suggests
it is the tenth. The centrum is again elongate, with a slight lateral
depression. The anterior face is perhaps slightly convex and 38% wider
than tall, while the posterior face is perhaps flat or concave. The
parapophyses are massive and positioned on the anteroventral corners.
There is a circular neural canal, a bit less than 40% as tall as the central
face. The prezygopophyses are quite massive compared to Elaphrosaurus,
though broken at their tips. The ventrolaterally projecting
diapophyses end much further from the parapophyses than in the third
cervical. The cervical ribs were apparently unfused to the
vertebrae. The neural spine is low and short craniocaudally, although only
its base remains. There is a large postzygopophyseal-central choana again,
but no step this time. There is no evidence of epipophyses.
A centrum is probably from the third dorsal, as the
parapophyses are partially on the centrum and partially on the neural arch, as
seen in the third dorsals of Elaphrosaurus and Dilophosaurus. There is a
slight lateral depression, the anterior face is 14% wider than tall and it looks
slightly opisthocoelous.
The other dorsal has a parapophysis placed on the
neural arch, but not at the dorsal edge of the prezygopophysis. This is
seen in the fourth dorsal of Elaphrosaurus. The centrum is again rather
elongate, with a small lateral depression and indeterminate face
convexity. The ventral edge is more strongly concave than
Elaphrosaurus. The diapophyses project laterally and appear
backswept. The prezygopophyses are very short, but still more massive than
Elaphrosaurus. A large postzygopophyseal-central choana is still
present, with a step like the third cervical. There are large
postzygopophyses and a moderate sized rectangular neural spine, with ventral
margins sloping towards the zygopophyses, especially the
postzygopophyses.
The caudal vertebra is probably from around the
fifth position, judging by elongation. The centrum has no lateral
depression and a moderately concave ventral surface. The anterior face is
perhaps concave while the posterior is slightly convex. There are
prominent transverse processes and the bases of large
prezygopophyses. The neural spine is craniocaudally expansive and there
looks to be a small anterior spine medial to the prezygopophyses.
Postzygopophyses are broken off.
The scapula is very broad for a theropod (~5.1
times longer than broad), which is more than even abelisaurs and megalosaurs
(~5.9 times). However, it lacks the expanded distal end of coelophysoids,
Dilophosaurus and basal non-theropod dinosaurs and is thus still
strap-like. Most of the anterior edge is lacking, but it is generally
similar to Carnotaurus, differing in the slightly concave posterior
margin. An extensive posteriorly facing glenoid is present.
The pelvis is unfused and propubic, with the pubis
about 27 degrees from the vertical. The preacetabular process is broken,
but extended past the pubic peduncle and is dorsoventrally narrow, less than
half of acetabular height. The dorsal edge of the ilium is fairly straight
and there is no vertical ridge or other ornamentation laterally. The large
pubic peduncle projects anteroventrally and is slightly expanded distally.
There is an extensive supracetabular shelf that ends halfway through the ischial
peduncle. The ischial peduncle is craniocaudally 42% as long as the pubic
peduncle. The postacetabular process is broken off, but could not have
been very tall.
Only the proximal section of the pubis is
preserved. The dorsal margin of an obturator fenestra can be seen,
although with the ventral margin incomplete, it could be an obturator
notch. There is no room for a pubic fenestra below it.
Only the most proximal section of the ilium is
preserved. Oddly, the ilial peduncle is much wider than the pubic
peduncle, contrasting with the peduncles they attach to. No conclusion
regarding obturator processes or flanges can be reached.
The femur is hollow and sigmoid in medial
view. The head is anteroposteriorly narrow, taking up less than half the
proximal end in medial view. The anterior trochantor is a bit more massive
than Dilophosaurus in proximal view and is hooked medially. There seems to
be a smaller process directly lateral to the anterior trochantor. A
trochanteric shelf is apparently absent. The fourth trochantor is smaller
than Dilophosaurus, but much larger than the reduced process in
Elaphrosaurus. Distally, the femur shows a very slight extensor groove and
a deep rounded flexor groove without a cruciate ridge. A mediodistal crest
is present, as in neoceratosaurs, but its proximal extent is hard to
judge.
The tibia is quite elongate (15% longer than the
femur) and slightly bowed laterally. The proximal end is craniocaudally
elongate, has a single cranial cnemial crest and the lateral condyle is smaller
than the medial condyle. The fibular crest cannot be
distinguished. Distally, the astragalus backed the tibia. In
distal view, the tibia is very anteroposteriorly narrow and roughly
triangular.
The tibia, astragalus and calcaneum were unfused to
each other. In the astragalus, the medial condyle is much larger, there
seems to be no transverse groove extending across the condyles and the condyles
are separated from the ascending process by a groove or excavation. The
extent of the ascending process is uncertain, as it is broken proximally.
However, it was obviously more prominent than ornithopods (contra Norman, 1990)
and a bit more extensive than Dilophosaurus, possibly making the astragalus
83-93% as tall as wide. The anterior concavity of the astragalus in distal
view is not as developed as coelurosaurs. The calcaneum was large and
rectangular.
The metatarsus, though elongate (61% of femoral
length), is not arctometatarsalian. Indeed, the third metatarsal is wider
proximally than distally. I am wary of He's pedal reconstruction however,
as metatarsal IV's distal end seems to be in lateral/medial view, as might
metatarsal III's. Also, metatarsal II's broken distal end appears to be in
posterior view. All this in a reconstruction of anterior view. Maybe
they were twisted due to post-burial deformation or illustrated
inaccurately. The proximal ends of metatarsals II and IV are flared
outward. In proximal view, metatarsal II is more tapered anteriorly,
metatarsal III more narrow, with a less expanded posterior end, and metatarsal
IV is wider and more wedge-shaped than in Elaphrosaurus. A phalanx,
identified as II-1, is shown backwards as its ginglymoid articulation is facing
proximally. This phalanx probably is II-1, as it is nearly identical in
comparative size and shape to that element in Elaphrosaurus. A series of
apparently articulated phalanges are identified as digit IV. IV-1 is the
largest and IV-2 is the shortest. They are all fairly similar in
morphology, with lateral ligament pits and ginglymoid distal articulations where
can be seen. The fact that IV-2 is the shortest leads to doubt they are
articulated correctly, as this is never seen in theropods. An ungual
phalanx is also preserved. It is short and gently curved, with no obvious
flexor tubercle.
Relationships-
Not many authors have attempted to classify
Chuandongocoelurus. He (1984) apparently assigned it to the Coeluridae,
though I cannot read the Chinese text. Such an assignment is obviously
based on small size, as the Coeluridae was the diminutive equivalent of the
Megalosauridae at the time. In actuality, Chuandongocoelurus is much more
primitive than Coelurus, although the fragmentary remains of the former and poor
description of the latter make this hard to demonstrate directly. The only
other author to hypothesize as to Chuandongocoelurus' relationships is Norman,
who referred it to Theropoda indet., while noting the primitively broad scapula,
low ascending process and uncompressed third metatarsal. As noted above,
although broad, the scapula is still strap-like. Additionally, it is much
broader than any other theropod or basal dinosaur and is thus an autapomorphy of
the genus. The low ascending process is actually broken, though it would
be of ceratosauroid level if complete (after Welles and Long, 1974). The
third metatarsal is compressed in proximal view, leading to an hourglass shape
seen in tetanurans and neoceratosaurs.
All material of Chuandongocoelurus is obviously
theropod, as seen by the hollow centra and femur, strap-like scapula, tall
ascending process, compressed third metatarsal and other characters. There
are three competing hypotheses for basal theropod phylogeny-
(((Coelop, Diloph) (Elaph (Cerato, Abeli)))
Tetan) Holtz 2000, Sereno 1999
(Coelop (Diloph (Tetan (Cerato (Elaph, Abeli)))))
Rauhut 1999
(Coelop (Abeli (Cerato,
Tetan)))) Carrano
and Sampson 1999
The latter two are unpublished, but Rauhut kindly
sent me his thesis, so I can use it to test Chuandongocoelurus' phylogenetic
relationships.
Using Holtz's (2000) phylogeny, Chuandongocoelurus
is a ceratosaur based on- ilial supracetabular shelf; metatarsal III proximal
area larger than II or IV. It would lack the ceratosaurian characters of-
ischial antitrochantor large; trochanteric shelf of femur well developed.
It also has the neoceratosaurian character of metatarsal III proximal surface
dumbbell shaped. It lacks the (Ceratosaurus + Abelisauroidea) characters-
cervical centra markedly opisthocoelous; fourth trochantor little
developed. However, it has the abelisauroid characters- mid-cervical
centra anterior faces more than 20% wider than tall; dorsal centra wider than
high; scapular blade at least four times longer than midshaft width;
iliac-ischial articulation smaller than iliac-pubic articulation. It also
has the tetanurine characters of- scapular blade at least four times longer than
midshaft width; iliac-ischial articulation smaller than iliac-pubic
articulation; femoral extensor groove present; trochanteric shelf absent; crista
fibularis present. It lacks the tetanurine character of an allosauroid
tarsus.
The supracetabular shelf may be a valid ceratosaur
synapomorphy, though it is also present in Herrerasaurus. Metatarsal III
has a proximal area greater than II and IV in basal tetanurans as well (Britt
1991, Dong and Currie 1993), so this is not a valid ceratosaurian
character. Chuandongocoelurus does appear to lack an ischial
antitrochantor, although this is very possibly due to the illustration
quality. The absence of a trochanteric shelf could easily mean this was a
gracile individual, as known individuals of Dilophosaurus for instance lack
trochanteric shelves. The presence of a dumbbell-shaped proximal
metatarsal III with anterior and posterior ends expanded to slightly overlap the
surfaces of metatarsals II and IV is a complex character. Both
Ceratosaurus and Elaphrosaurus show strongly expanded posterior ends that
overlap II and IV, but the anterior ends are expanded very slightly in
Elaphrosaurus and vary from expanded moderately (IV) to the opposite condition
(II) in Ceratosaurus. Chuandongocoelurus has more of a dumbbell shape than
Ceratosaurus and resembles Elaphrosaurus except the lack of a posterior
expansion behind IV. Further examination of this character is beyond the
scope of this post. The fourth trochantor of Elaphrosaurus is reduced
(Janensch, 1925), but not that of Ceratosaurus (Madsen and Welles, 2000), so I
am confused by this character defining a (Ceratosaurus + Abelisauroidea) clade
excluding Elaphrosaurus. As Elaphrosaurus has all cervical centra at
least 20% wider than tall (except the seventh- 11%), all dorsal centra wider
than tall (except the fourth- 97%) and a craniocaudally longer pubic peduncle
than ischial peduncle (the scapula is unknown), I think its lack of
opisthocoelous cervicals is more likely to be a reversal. The tetanurine
characters present in Chuandongocoelurus are also abelisauroid synapomorphies
(discussed above), known in neoceratosaurians (extensor groove- Rauhut, 1999),
gracile ceratosaurs (trochanteric shelf absent) or are present in at least some
ceratosaurs (crista tibiofibularis present). Thus, although somewhat
problematic, Holtz's analysis supports Chuandongocoelurus as an abelisauroid,
assuming Elaphrosaurus is in this clade (as it was in Holtz, 1994).
In Sereno's (1999) phylogeny, Chuandongocoelurus
would not be a ceratosaur because- pelvis not fused; ischial antitrochantor
smaller than articular surface for ilium; femoral trochanteric shelf not
trough-shaped; astragalocalcaneum unfused. It is not coelophysid, as it
has dorsal centra less than 2.5 times as long as tall, and is not coelophysine,
as it has subrectangular dorsal neural spines. It has the tetanurine
characters of an iliopubic articulation anteroposteriorly longer than the
ilioischial articulation and a third metatarsal with proximal width less than
metatarsals II or IV at its narrowest point. It lacks the neotetanurine
character of prominent hypapophyses on anterior dorsals. It has the
coelurosaurian characters postaxial cervical centra not strongly opisthocoelous
and no transverse groove across astragalar condyles, but lacks a medial
ascending process edge at a high angle.
The lack of fusion could be due to the subadult age
of Chuandongocoelurus, and the trochanteric shelf and ischial antitrochantor
were discussed above. The large iliopubic articulation is also seen in
abelisauroids, as mentioned above. The narrowness of metatarsal III
proximally is also seen in Dilophosaurus, so this is not unheard of in
ceratosaurs. Elaphrosaurus has strongly amphicoelous cervicals and
Dilophosaurus lacks a transverse astragalar groove, so these characters are not
unique to coelurosaurs. Thus, Sereno's analysis presents weak evidence
against a coelophysid or neotetanurine identity.
In Rauhut's (1999) phylogeny, Chuandongocoelurus
would be a member of the (Dilophosaurus + Ceratosauria + Tetanurae) clade based
on- anterior faces of cervical centra much wider than tall. It would be a
member of the (Ceratosauria + Tetanurae) clade based on- femoral distal condyles
well rounded in lateral view; distal tibia strongly expanded mediolaterally and
braodly triangular. However, it lacks a substantial femoral extensor
groove, unlike this clade. It is a ceratosaur (equivalent to traditional
Ceratosauroidea or Neoceratosauria) based on- scapula not expanded
distally. It differs from ceratosaurs in lacking a transverse groove
across the astragalar condyles. It shares the following characters with
Elaphrosaurus- dorsal pleurocoels absent; mid-caudal neural spines
subrectangular and sheet-like. It is not tetanurine based on- fibular
condyle of tibia not offset from cnemial crest; ascending process lower than
astragalar body.
The presence of extensor grooves in theropods seems
to be sporadic. They are absent in Liliensternus, Dilophosaurus
and spinosaurids (as well as some coelurosaurs), but present in Segisaurus,
Syntarsus, ceratosaurs and most basal tetanurans. Thus, I don't think we
can conclusively say its presence is a synapomorphy of a Ceratosauria+Tetanurae
clade. Although unlike other ceratosaurs in lacking a strong extensor
groove, the difference between Liliensternus and Syntarsus for this character
shows it can vary within closely related taxa. The absence of a transverse
astragalar groove may be due to the illustration quality, as only a few
horizontal lines are used to indicate the texture in this area. Rauhut's
characters also leave many questions, though Chuandongocoelurus seems close to
Elaphrosaurus. Entering Chuandongocoelurus into Rauhut's matrix results in
22176 trees, with Chuandongocoelurus a member of the (Ceratosauria + Tetanurae)
clade, but excluded from the Carnosauria and (Coelurus + Compsognathidae +
Tyrannoraptora) clade. As carnosaurs include megalosaurs and spinosaurs in
Rauhut's trees, this leaves the Ceratosauria, extremely basal Tetanurae (along
with Piatnitzkysaurus and "Szechuanosaurus" zigongensis) and extremely basal
Coelurosauria (along with Proceratosauria).
It seems there are still some problems to work out
with basal theropod relationships (work in progress...), but the above analyses
all seem to place Chuandongocoelurus in the vacinity of
abelisauroids.
Abelisauroid characters- cervical neural spines
project only slightly above neural arch; preacetabular process less than 1.5
times as high as acetabulum; on ilium, ischial peduncle much narrower
craniocaudally than pubic peduncle.
Compared to Ceratosaurus and Carnotaurus,
Elaphrosaurus and Chuandongocoelurus share the following synapomorphies-
elongate anterior dorsal centra (posterior central face height
<65% of central length); anterior cervicals with low rounded neural
spines; proximal caudal neural spines elongate anteroposteriorly, extending
over ~2/3 of central length; femoral shaft strongly sigmoid; mediodistal crest
of femur extends posterodistally across medial surface, not along anteromedial
edge; deep groove posteriorly between lateral and medial tibial condyles;
laterally hooked tip of cnemial crest.
Xenotarsosaurus also lacks the hindlimb
characters. Coelophysids developed elongate anterior dorsal centra and
very low rounded cervical neural spines in parallel, as they are absent in
Herrerasaurus, Dilophosaurus and basal tetanurines. Herrerasaurids also
developed long proximal caudal neural spines. The last two characters are
also developed in tetanurines.
Based on other evidence, Ligabueino, Masiakasaurus,
Noasaurus, Velocisaurus and probably Elaphrosaurus form a monophyletic group,
perhaps best termed the Noasauridae. The lack of comparable material in
many taxa makes their interrelationships difficult to establish, but characters
shared by some include the hyperextendable second pedal digit of Ligabueino and
Noasaurus, the tapered preacetabular process of Elaphrosaurus and Ligabueino,
and especially the reduced lateral metatarsals of Masiakasaurus, Noasaurus and
Velocisaurus (also seen to a lesser degree in the fourth of
Elaphrosaurus). Their poor preservation and description makes it difficult
to evauluate if they share any of the Chuandongocoelurus +
Elaphrosaurus synapomorphies, but Masiakasaurus has low
rounded anterior cervical neural spines and elongate anterior dorsal
centra. Ligabueino lacks a strongly sigmoid femur and the derived
mediodistal crest morphology.
Chuandongocoelurus differs from Elaphrosaurus
in several ways- large postzygopophyseal-central choana in presacral
vertebrae; step in postzygopophyseal-central choana of anterior cervicals and
anterior dorsals; lateral depression of third cervical extends over 60% of
central length; posteroventral border of anterior cervical centra not convex;
larger prezygopophyses on posterior cervicals to proximal caudals; small
anterodorsally projecting process anterior to proximal caudal neural spines;
preacetabular process less than half acetabular height; pubic peduncle expanded
distally; postacetabular process narrower; obturator notch (or foramen) in
pubis; ilial peduncle of ischium larger than pubic peduncle; long dorsoventral
axis of femoral head angled less anterodorsally; prominent fourth trochantor;
medial condyle larger than lateral condyle on tibia; fibular crest indistinct;
no transverse groove across astragalar condyles; outer edges of metatarsals II
and IV flared priximally; proximal end of metatarsal III wider than distal end;
metatarsal IV broader than II proximally; metatarsal III without lateroplantar
expansion to back IV.
Many of these are plesiomorphic or found in other
theropods, but several are unique to Chuandongocoelurus and are listed above in
the diagnosis. Thus, Chuandongocoelurus is not indeterminate, as Norman
(1990 and Rauhut (1999) have suggested.
In conclusion, Chuandongocoelurus is a
neoceratosaur most closely related to Elaphrosaurus, although how closely
related other noasaurids are to either of them is difficult to determine.
Perhaps Chuandongocoelurus and Elaphrosaurus should be classified as noasaurids,
however I would like more work done on abelisauroid relationships before such
taxonomic decisions are made. The presence of early neoceratosaurs in Asia
in not entirely unexpected, as Dandakosaurus and Lukousaurus may also be early
representatives.
References- He, 1984. The vertebrate fossils of Sichuan: Sichuan Scientific
and Technological Publishing House, 168 pgs.
Norman, David B. 1990. Problematic Theropoda: ``Coelurosaurs''.
p. 280-305 in David B. Weishampel, et al. (eds.), The Dinosauria. University of
California Press, Berkeley, Los Angeles, Oxford.
Anyone who wants figures of Chuandongocoelurus (3 pages), contact me
offlist. What to do next?
Mickey Mortimer |