Agilisaurus and the origin of the Pachycephalosauridae

Stegoceras validum (Lambe 1902; Late Cretaceous, Late Campanian; 75mya; 2m length; Fig. 1) was a basal dome-head dinosaur, or pachycephalosaur. Traditionally pachycephalosaurs have been linked to to stegosaurs, like Stegosaurus, troodontids like Sinornithoides, and ceratopsians, like Triceratops, but those are not supported in the large reptile tree (subset Fig. 4).

In the large reptile tree
Stegoceras was recovered as a sister to Agilisaurus (Peng 1990; ZDM 6011; Middle Jurassic; 1.2m length; Figs. 2-4).

In Stegoceras
the nares face somewhat forward, as in Agilisaurus. Similarly the forelimbs are tiny on this biped. The palpebral bones are incorporated into the skull itself. The antorbital fenestra is no longer visible. The dorsal and caudal ribs are quite wide, giving this dinosaur a wider than deep torso and tail first noted by Greg Paul, who kindly provided permission for his famous reconstruction (Fig. 1). The posterior tail is stiffened with ossified tendons originally thought to be gastralia.

Figure 1. Stegoceras, a basal pachycephalosaur from the Mid-Cretaceous is derived from a sister to Agilisaurus.

Figure 1. Stegoceras, a basal pachycephalosaur from the Mid-Cretaceous is derived from a sister to Agilisaurus.

Sullivan 2003 writes,
“Pachycephalosaurian dinosarus, known primarily fro their unusually thickened crania, are perhaps the most enigmatic and poorly understood dinosaurs.” Sullivan, like many traditional paleontologist, used ceratopsids for his phylogenetic outgroup. Traditionally pachycephalosaurs and ceratopsids have been lumped in the clade “Marginocephalia” (Sereno 1986). The large reptile tree (subset Fig. 3) does not support that nesting. Instead, the odd Agilisaurus nests with Stegoceras. It shares many traits including incipient anteriorly facing nares, small fore limbs and a long tail. The presence of upper temporal fenestrae in Stegoceras,though tiny, mark this as a basal pachycephlosaur.

Sereno (1986)
based the taxon on four synapomorphies (listed before the publication of Agilisaurus, which does or could share all 4 traits):

  1. narrow parietal shelf
  2. posterior squamosal shelf
  3. short posterior premaxillary palate
  4. short postpubic process (the original retroverted pubis sans the prepubic process)

The most basal member of the Marginocephalia
is reported to be Stenopelix, which we looked at earlier here. With current data,
the clade “Marginocephalia” has no utility because pachycephalosaurs do not nest with ceratopsians to the exclusion of all other taxa.

Sullivan continues
“It is clear that pachycephalosaurids appear rather abruptly in the fossil record (the Santonian). The origin of this group, and the directionality in dispersals of its taxa can only be speculative based on current (2003) information.”

Figure 1. The skull of Agilisaurus (Late Jurassic) provides the bauplan for the skull of more derived pachycephlosaurs, like Stegoceras.

Figure 1. The skull of Agilisaurus (Late Jurassic) provides the bauplan for the skull of more derived pachycephlosaurs, like Stegoceras. Note the anteriorly facing nares. The palpebral bone is in two parts here.

Agilisaurus is an ornithischian oddball.
And, as in other phylogenetic enigmas, like Longisquama and Sharovipteryx, the oddballs (in this case, Agilisaurus + pachycephlosaurs) nest together. The enigmatic structures suddenly become synapomorphies when sister taxa are found to share apparent autapomorphic (unique) traits.

Figure 3. Agilisaurus, like Stegoceras, was a biped with tiny forelimbs and a long tail, providing the blueprint for later pachycephalosaurs.

Figure 3. Agilisaurus, like Stegoceras, was a biped with tiny forelimbs and a long tail, providing the blueprint for later pachycephalosaurs. Note the broad fronts and tiny parietals.

The large and broad frontals
of Agilisaurus, together with the relatively small parietals are precursor traits to the dome skulls of pachypleurosaurs. At this point, and with the limited number of taxa in the ornithischian subset of the large reptile tree, this is how relationships are recovered. Xu et al. 2006 in their paper on Yinlong, recovered Agilisaurus basal to heterodontosaurs in the branch leading to their “Marginocephalia.”

Figure 4. The phytodinosauria. Here Stegoceras and the pachycephalosaurs nest with the Middle Jurassic Agilisaurus.

Figure 4. The phytodinosauria. Here Stegoceras and the pachycephalosaurs nest with the Middle Jurassic Agilisaurus.

One of the problems traditional paleontologists have
with the Ornithischia is they don’t know which taxa are basal. They often use Lesothosaurus, rather than Chilesaurus and Daemosaurus as a basal taxon. Here Lesothosaurus is basal to Stegosaurus through Scutellosaurus. We talked about Chilesaurus earlier here. Traditional paleontologists don’t recognize the clade Phytodinosauria, either. When they do, everything will become clear.

I’d like to know more about
Micropachycephylosaurus, a tiny taxon with a long name, reportedly close to the origin of the Ceratopsia, but I need data.

References
Barrett PM, Butler RJ and Knoll F 2005. Small-bodied ornithischian dinosaurs from the Middle Jurassic of Sichuan, China. Journal of Vertebrate Paleontology 25:823-834.
Currie PJ and Padian K 1997. Encyclopedia if Dinosaurs. Academic Press.
Dodson P. 1990. Marginocephalia. Pp. 562-563 in The Dinosauria (Weishampel DB, Dodson P and Osmólska H, eds.) University of California Press, Berkeley.
Lambe LM 1902. New genera and species from the Belly River series (Mid-Cretaceous). Contributions to Canadian Paleontology. Geological Survey of Canada 3:25-81.
Lambe LM 1918. The Cretaceous genus Stegoceras, typifying a new family referred provisionally to the Stegosauria. Transactions of the Royal Society of Canada. 12(4):23-36. Peng G-Z 1990. New small ornithopod (Agilisaurus louderbacki gen. et sp. nov.) from Zigong, China. Newsletter of the Zigong Dinosaur Museum 2: 19–27.
Peng G-Z 1992. Jurassic ornithopod Agilisaurus louderbacki (Ornithopoda: Fabrosauridae) from Zigong, Sichuan, China. Translated by Will Downs. Vertebrata Palasiatica 30: 39-51.
Sullivan RM 2003. Revision of the dinosaur Stegoceras Lambe (Ornithischia, Pachycephalosauridae). Journal of Vertebrate Paleontology 23 (1): 181–207.
Xu X, Forster CA, Clark JM and Mo J 2006. A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences 273: 2135–40. doi:10.1098/rspb.2006.3566. PMC 1635516. PMID 16901832.

wiki/Agilisaurus
wiki/Stegoceras

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