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My next foray into the Ornithischia. Thanks
to both Jaime Headden and Pete Buchholz for helping me with taxa I lack
references to, as well as to Alessandro Marisa for his helpful list of
marginocephalian characters.
Jeholosaurus Xu, Wang and You 2000
J. shangyuanensis Xu, Wang and You
2000
Etymology- "lizard from Jehol, Shangyuan", the
first referring to the old geographical name for western Liaoning and northern
Hebei, the second for a larger geographic locality.
Barremian, Early Cretaceous
Yixian Formation, Liaoning, China
Holotype- (IVPP V 12529) incomplete skull (63 mm),
mandibles (59 mm), cervical vertebrae, caudal vertebrae, femur (90 mm), tibia
(107 mm), fibula, metatarsal I (24 mm), metatarsal II, metatarsal III (55 mm),
metatarsal IV, pedal phalanges
Referred- (IVPP V 12530) incomplete skull (~50 mm),
cervical vertebrae
Diagnosis- enlarged laterodorsal nasal foramina;
quadratojugal fenestra more than 25% maximum quadratojugal length;
quadratojugal less than 30% of skull height; predentary almost 150% of
premaxillary body length; dentary extends posteriorly almost to posterior border
of angular; metatarsal III placed more anteriorly than other
metatarsals.
Description- These specimens were found in 2000
from the tuffaceous sandstone of the first member of the lower Yixian
Formation. The paper is only a preliminary description. In
depth study and phylogenetic analysis are said to be published later.
The extremely large orbits and short snout lead me to believe they may be from
juveniles, but I'm not certain.
The holotype skull is crushed dorsoventrally, but
the referred specimen is not, although the anteriormost portion is
missing. The premaxilla resembles Hypsilophodon, with a fairly elongate
subnarial process not quite contacting the lacrimal. Posteroventrally, the
subnarial process is deeply concave, seen in hypsilophodonts and a few other
taxa. The external naris is as long as the main premaxillary body.
The ventral edge is level with the maxilla's ventral edge and a subnarial
foramen is developed between the bones. The former is a primitive
character also seen in Lesothosaurus, Bugenasaura and Lesothosaurus. The
latter is often used as a saurischian synapomorphy, but is seen in Hypsilophodon
as well for instance. Six teeth extend the length of the premaxilla, so
that not even the most anterior part is edentulous. This is the same
number as Lesothosaurus, but the latter condition is more basal than even that
taxon, though Bugenasaura and Agilisaurus exhibit it. These teeth are
slender and recurved without denticles, similar to Zephyrosaurus and
Thescelosaurus. The maxilla has an antorbital fossa about 14% of
cranial length and a small antorbital fenestra located posteriorly.
This is similar to the condition of Hypsilophodon and basal iguanodonts.
The deeply inset ventral margin suggests cheeks may have been present, if they
were in any ornithischian. At least thirteen maxillary teeth are
present. The nasals have large foramina dorsolaterally and are said to
have a midline fossa. The figures make it look like there are paired nasal
fossae separated by the midline, so I am uncertain about this. A midline
nasal fossa may also be present in Agilisaurus and "Yandusaurus"
multidens. No palpebrals are preserved. The jugal does not contact
the antorbital fossa, as in most ornithopods. It has a slender posterior
process, as the laterotemporal fenestra is tall, similar to Agilisaurus.
The posterior jugal process expands distally, similar to dryomorphans and
"Yandusaurus" multidens. The postorbital and frontal are typical of
basal hypsilophodonts. The quadratojugal resembles Agilisaurus more than
Lesothosaurus, Hypsilophodon or "Y." multidens, being small and confined to the
posteroventral corner of the skull. There is a very short anterior process
and a small dorsal component that barely contacts the laterotemporal
fenestra. A round fenestra penetrates the bone, as in Hypsilophodon.
The quadrates ventral end is almost at the level of the maxillary tooth
row, like that of Lesothosaurus and "Yandusaurus" multidens.
The mandible lacks an external mandibular fenestra,
like most ornithischians baring Lesothosaurus, stegosaurs and
Heterodontosaurus. The predentary has an elongate ventral process twice as
long as the body and lateral process combined. This is unique to
Jeholosaurus and Lesothosaurus as far as I can tell. The angular is very
narrow and the dentary extends posteriorly dorsal to it almost to the posterior
mandibular edge. The coronoid process is covered by the jugal, so cannot
be examined.
Maxillary teeth have six to nine denticles and
ridges corresponding to them, a common hypsilophodont trait. Posterior
teeth have wear facets and are larger and more slender than anterior
teeth. They also have flatter sides and more prominent ridges.
Dentary teeth have about eight denticles and weak ridges.
Cervical centra are keeled ventrally and no
ossified tendons "have been observed".
The bowed femur has an anterior trochantor slightly
lower than the greater trochantor and a third as wide as the latter. They
are separated by a shallow cleft and the extensor groove is absent, unlike
iguanodonts. The fourth trochantor is probably pendent. Within the
metatarsus, metatarsal III is placed anterior to the others and metatarsal I
posterior. Metatarsal I is reduced to a splint. The pedal phalangeal
formula is 2-3-4-5-0.
Relationships-
The authors are undecided as to the relationships
of Jeholosaurus. They consider it an ornithopod, citing several ornithopod
characters from Sereno's 1999 analysis. However, they then list various
ornithopod and cerapod characters lacking in the taxon. Finally, they cite
a few characters also seen in Agilisaurus, "Y." multidens and Xiaosaurus,
suggesting these taxa might form a monophyletic assemblage of Chinese
ornithopods. Clearly these mixed conclusions are not
satisfactory.
Unfortunately, analyses of basal ornithopods are
scarce. Increasing evidence indicates hypsilophodonts are paraphyletic,
but their relationships have yet to be analyzed in depth. Several experts
are working on this problem, but the results are unpublished. Weishampel
and Heinrich (1992) is the most recent analysis of hypsilophodonts I have access
to (11 taxa, 37 characters). Examination of the matrix revealed some
problems, such as several characters with the incorrect polarity. This is
due to the use of Heterodontosaurus as an outgroup, in addition to a
hypothetical outgroup with the same states as Heterodontosaurus. To solve
this, I replaced the hypothetical ancestor with Lesothosaurus and reversed the
polarity of seven characters. I found twelve characters to be invalid
for various reasons and modified four others to be more accurate. There
were several errors in the matrix (coding Heterodontosaurus as having maxillary
teeth less than 150% as tall as wide and a scapula shorter than the humerus;
coding Camptosaurus as having a naris more than 20% of cranial length; coding
"Yandusaurus" multidens as having jugal participation in the antorbital fossa,
lacking quadratojugal-squamosal contact; coding Dryosaurus as having paired
frontals wider than long, etc.), but not nearly as many as Xu et al's (2001)
matrix. I then added twenty-nine more characters from the literature,
personal observation and posts to the list. This brought the number of
characters to 54. More taxa were also added, resulting in a total of
26. I ran the matrix in PAUP, with the multistate characters ordered (it
was appropriate) and 52 MPT's of 170 steps were found. The consensus
tree is as follows-
+--Lesothosaurus
`--+--+--Scelidosaurus
|
`--Huayangosaurus
`--+--+--Rhabdodon
| `--+--Agilisaurus
| `--+--Thescelosaurus
|
`--+--"Yandusaurus" multidens
|
|--Othnielia
|
|--+--Parksosaurus
|
| `--Gasparinisaura
|
|--Hypsilophodon
|
|--+--Zephyrosaurus
| | `--Orodromeus
|
`--Jeholosaurus
`--+--Bugenasaura
`--+--Yandusaurus
`--+--Heterodontosaurus
`--+--+--Psittacosaurus
| `--Neoceratopsia
`--+--Pachycephalosauria
`--+--Tenontosaurus dossi
|--Tenontosaurus tilletorum
`--+--Muttaburrasaurus
`--+--Dryosaurus
|--Camptosaurus
`--Iguanodon
Here I would like to point out that deleting a
single character or taxon in this analysis can make hypsilophodonts paraphyletic
or polyphyletic. Similarily, the topology within the clade can fully
resolve in several different ways although there are patterns that are common
(Rhabdodon basal; Yandusaurus+Agilisaurus; Parksosaurus+Gasparinisaura;
Zephyrosaurus+Orodromeus). Jeholosaurus is often times placed by a
Zephyrosaurus+Orodromeus clade, "Yandusaurus" multidens or Othnielia.
Another thing I would like to comment on is the marginocephalian-iguanodont
clade. All previously proposed ornithopod synapomorphies were used (when
valid) and many combinations of deleted taxa were tried, but this clade
persisted. This is contradictory to all previous analyses I'm aware of,
but the synapomorphies seem valid enough. One thing I discovered while
performing this analysis is that most proposed synapomorphies of higher
ornithischian clades are seen in other groups, reversed often in the clade or
completely invalid. Although Thyreophora came out monophyletic, I used up
all the synapomorphies I could code to make it so. Marginocephalia is
often paraphyletic, as seen above, although ceratopsians, pachycephalosaurs and
heterodontosaurids are always close. Again, this is despite my efforts to
include all proposed marginocephalian synapomorphies, along with those linking
heterodontosaurs with various marginocephalians. Similarily, Ornithopoda
never appears, though only one more step is needed to make it monophyletic (9
MPT's, 171 steps). In this constraint tree that forces ornithopod
monophyly, Marginocephalia is also monophyletic. Rhabdomorphs are also
very close. I personally think it is a more probable phylogeny, and it is
illustrated below.
+--Lesothosaurus
`--+--+--Scelidosaurus
|
`--Huayangosaurus
`--+--+--Heterodontosaurus
| `--+--+--Psittacosaurus
| |
`--Neoceratopsia
| `--Pachycephalosauria
`--+--+--Thescelosaurus
| `--+--Agilisaurus
| `--+--"Yandusaurus"
multidens
|
|--Othnielia
| |--+--Parksosaurus
| |
`--Gasparinisaura
|
|--Bugenasaura
|
|--Hypsilophodon
|
`--+--+--Zephyrosaurus
| | `--Orodromeus
| `--Jeholosaurus
`--+--Rhabdodon
`--+--Yandusaurus
`--+--Tenontosaurus
tilletorum
`--+--Tenontosaurus
dossi
`--+--Muttaburrasaurus
`--+--Iguanodon
|--Camptosaurus
`--Dryosaurus
Regardless of what relationships are higher in the
Ornithischia, Jeholosaurus seems to be most closely related to taxa generally
referred to as hypsilophodontids. The relationship between Jeholosaurus
and the Zephyrosaurus-Orodromeus clade is very weak, only supported by the
absence of premaxillary tooth serrations in Jeholosaurus and
Zephyrosaurus. In fact, virtually all the characters diagnosing clades
within the Hypsilophodontia are prone to homoplasy and known only in few
members, so I wouldn't place high degrees of confidence in any of
them. I suggest that more work be done on ornithischian phylogeny,
especially the hypsilophodont section of the tree. Supposedly well
established clades, such as Euornithopoda, Euiguanodontia and
Marginocephalia look somewhat questionable. However, Genasauria, Cerapoda,
Ceratopsia, Iguanodontia and Dryomorpha seem well supported. Jeholosaurus
is located somewhere within the hypsilophodont nexus, but more precise placement
must wait for more detailed studies. Relations to Orodromeus,
"Yandusaurus" multidens, Othnielia and Zephyrosaurus are suggested
however.
Reference- Xu, Wang and You, 2000. A primitive
ornithopod from the Early Cretaceous Yixian Formation of Liaoning. Vertebrata
PalAsiatica 38(4) 318-325. If anyone wants the figures of the holotype and
paratype skulls, request them offlist. I'm suffering from theropod
withdrawl looking at all these ornithischians for the past few days.
Expect the next details segment to be about something a bit closer to
birds.....
Mickey
Mortimer |