Extreme cranial ontogeny in the upper cretaceous dinosaur pachycephalosaurus

Abstract

Background: Extended neoteny and late stage allometric growth increase morphological disparity between growth stages in at least some dinosaurs. Coupled with relatively low dinosaur density in the Upper Cretaceous of North America, ontogenetic transformational representatives are often difficult to distinguish. For example, many hadrosaurids previously reported to represent relatively small lambeosaurine species were demonstrated to be juveniles of the larger taxa. Marginocephalians (pachycephalosaurids + ceratopsids) undergo comparable and extreme cranial morphological change during ontogeny.

Methodology/principal findings: Cranial histology, morphology and computer tomography reveal patterns of internal skull development that show the purported diagnostic characters for the pachycephalosaurids Dracorex hogwartsia and Stygimoloch spinifer are ontogenetically derived features. Coronal histological sections of the frontoparietal dome of an adult Pachycephalosaurus wyomingensis reveal a dense structure composed of metaplastic bone with a variety of extremely fibrous and acellular tissue. Coronal histological sections and computer tomography of a skull and frontoparietal dome of Stygimoloch spinifer reveal an open intrafrontal suture indicative of a subadult stage of development. These dinosaurs employed metaplasia to rapidly grow and change the size and shape of their horns, cranial ornaments and frontoparietal domes, resulting in extreme cranial alterations during late stages of growth. We propose that Dracorex hogwartsia, Stygimoloch spinifer and Pachycephalosaurus wyomingensis are the same taxon and represent an ontogenetic series united by shared morphology and increasing skull length.

Conclusions/significance: Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are reinterpreted as younger growth stages of Pachycephalosaurus wyomingensis (adult). This synonymy reduces the number of pachycephalosaurid taxa from the Upper Cretaceous of North America and demonstrates the importance of cranial ontogeny in evaluating dinosaur diversity and taxonomy. These growth stages reflect a continuum rather than specific developmental steps defined by "known" terminal morphologies.

Figure 1. The distinctive squamosal ornamentation and relatively high, narrow frontoparietal dome of Stygimoloch spinifer.

The holotype left squamosal (UCMP 119433) in (A) and a right squamosal (UCMP 131163) in (B) in posterior view. UCMP 131163 was found associated with the relatively high, narrow frontoparietal dome in right lateral view in (C). The frontoparietal suture is highlighted in white. The intrafrontal suture is not visible on the dorsal surface. Numbering sequence (1–3) of horns and nodes in (A) and (B) from Galton and Sues . Scale bars are 5 cm.

Figure 2. The skull of Stygimoloch spinifer, MPM 8111, in right lateral view.

This skull revealed the low angle orientation of horn #1 and surrounding nodes on the elongated squamosal shelf. Scale bar is 5 cm.

Figure 3. Cranial ontogenetic sequence of Pachycephalosaurus wyomingensis with morphological landmarks highlighted in color.

The ontogenetically oldest adult, AMNH 1696, in (A) dorsal and (B) right lateral views. A younger adult, UCMP 556078 (cast) with inflation of the frontoparietal dome+lateral cranial elements and mature nasal and squamosal nodal ornamentation in (C) dorsal and (D) right lateral views. MPM 8111, a partial skull of “Stygimoloch” in (E) dorsal and (F) left lateral views (reversed) illustrates the high narrow frontoparietal dome, squamosal nodes and horns characteristic of the subadult growth stage. Landmarks on the dorsal skull of MPM 8111 in orange (anterior) and red (posterior) constrain the position of the dome. The youngest growth stage in this cranial ontogenetic series is “Dracorex”, TCNI 2004.17.1 (cast) in (G) dorsal and (H) right lateral views. The position of the squamosal horns and nasal nodes are consistent in these four pachycephalosaurid skulls, which increase in overall length and size from youngest (G,H) to oldest (A,B). Scale bar is 5 cm.

Figure 4. A coronal CT scan through the dome of “Stygimoloch” (MPM 8111).

The intrafrontal suture (black arrow) is open internally supporting the subadult status of this pachycephalosaurid. The braincase (bc) and foramen magnum (fm) are clearly visible.

Figure 5. A composite image of “Stygimoloch” (MPM 8111) and “Dracorex” (TCNI 2004.17.1; cast) illustrates the subadult cranial ontogenetic stage morphology.

The composite is constrained by landmarks on the skulls showing the position of the inflated frontoparietal dome of “Stygimoloch” on the posterior dorsal surface of “Dracorex” in (A) dorsal and (B) right lateral views. Scale bars are 5 cm.

Figure 6. The holotype adult skull of Pachycephalosaurus “reinheimeri” (DMNS 469).

(A) Asymmetrical clusters of massive, slightly pointed to rounded nodal ornaments on the dorsal surface of the squamosals dominate the posterior skull. (B) The squamosal nodes in posterior view. Scale bars are 5 cm.

Figure 7. An adult Pachycephalosaurus skull (VRD 13) with ontogenetically advanced squamosal ornamentation.

The adult cranial morphology and clusters of squamosal nodes in (A) and a close-up of the right squamosal ornamentation in (B) in right lateral views.

Figure 8. Craniohistological coronal sections of “Stygimoloch” (A–C) and Pachycephalosaurus (D–F) skulls.

(A) UCMP 128383.PSF-3, a coronal section through the frontoparietal dome clearly shows the open intrafrontal suture (white arrow) in this subadult “Stygimoloch”. Scale bar is 2 cm. (B) UCMP 128383.PSF-3, a view of the middle region of the frontoparietal dome with highly vascularized tissue and vascular spaces oriented in a radial pattern. Scale bar is 1 mm. (C) UCMP 128383.PSF-3, the dorsal-most region of the frontoparietal dome reveals abundant fibers and fibrocytes present when the frontoparietal dome inflates and ossifies. Scale bar is 100 µm. (D) Coronal section of “Pachycephalosaurus”, VRD 13. The denser and less vascularized tissue throughout this late growth stage suggests subsidence of frontoparietal dome growth. Black arrow points to postorbital nodal ornament sectioned in (F) and (G). Scale bar is 2 cm. (E) VRD 13, the ends of fibers (white arrows) extend into the outermost margin of the interior of the dome and likely connected to a fibrous epidermal covering. Scale bar is 100 µm. (F) VRD 13, a postorbital nodal ornament in transverse section lacks any evidence of an epidermal attachment or growth dorsally. Scale bar is 1 mm. (G) VRD 13, the erosional surface (white arrows) points to continued modification in size and shape of this postorbital nodal ornament. No evidence of an epidermal attachment or growth dorsally in the degraded surface preserved in the overlying tissue. Scale bar is 100 µm.

Figure 9. Histological sections of “Stygimoloch” squamosal horns.

(A) UCMP 128383.CrSp.1–2, transverse section of a highly-vascularized, expanding “Stygimoloch” squamosal horn found associated with a frontoparietal dome (see Fig. 3A) and partial skeleton. Scale bar is 2 cm. (B) UCMP 128383.CrSp.1–2, the red arrows point to the interior depositional surface and expansion of this squamosal horn. This metaplastic tissue closely resembles bone that contacts a periosteum, but the absence of osteocytes and abundant fibers indicates the horn is expanding through metaplasia rather than periosteal ossification. Scale bar is 100 µm. (C,D) UCMP 119433.SqSp.1–4, transverse sections reveal metaplastic tissue in the largest squamosal horn from the holotype of “Stygimoloch” (UCMP 119433). In (C), the interior erosional line is interrupted by a depositional surface along the red arrows. Scale bar is 2 cm. In (D), the erosional surface (red arrow) indicates the horns are getting smaller. Scale bar is 100 µm.

Figure 10. Histological sections of “Stygimoloch” squamosal horn.

(A–C), MOR 560.CrSp.1-L2, a squamosal horn in erosion mode in (A) longitudinal and (B,C) transverse section becomes smaller. The red arrows in (C) point to a surface that is degrading with no sign of metaplasia with a thin metaplastic veneer. Scale bar is 2 cm in (A) and 1 mm in (B) and (C).