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.
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-
| `--+--"Yandusaurus" multidens
| | `--Gasparinisaura
| | `--Orodromeus
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.
| | `--Neoceratopsia
| `--+--"Yandusaurus" multidens
| | `--Gasparinisaura
| | `--Orodromeus
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.....