The specimen was collected in Carter County, Montana, USA, approximately 60–70 meters above the base of the Hell Creek Formation (Latest Maastrichtian), which averages 150 meters in thickness in southeastern Montana ^[14]. Exact coordinates of the collection site are on file at the Burpee Museum of Natural History (BMR). The frontoparietal dome was found at the base of a mudstone butte; no other skeletal remains were recovered.

The specimen, BMR P2001.4.5, consists of an isolated frontoparietal dome (Figure 1). The specimen measures 620 mm in circumference, 310 mm in length from the rostral end to the posterior margin of the caudal region, and 132 mm in height. This is consistent with the average dome dimesions of the Maastrichtian pachycephalosaurid Pachycephalosaurus wyomingensis, in which adult frontoparietal domes average between 260 and 360 mm in length ^[15]. The frontoparietal suture is present on the right lateral side beneath the cortical surface (Figure 1D), though no suture is apparent on the dorsal surface of the specimen.

Four nominal taxa of pachycephalosaurids have been described from the Hell Creek Formation: Pachycephalosaurus wyomingensis (Brown and Schlaikjer, 1943) ^[16], Stygimoloch spinifer (Galton and Sues, 1983) ^[17], Dracorex hogwartsia (Bakker et al., 2006) ^[18], and Sphaerotholus buchholtzae (Williamson and Carr, 2002) ^[19]. The dorsal surface of BMR P2001.4.5 is smooth, which is a characteristic of the largest pachycephalosaurid specimens ^[16] and lacks the distinctive, regular dimpling found in Sphaerothlous or polygonal sulci present on smaller domes ^[19]. Considering its large size, BMR P2001.4.5 is too broad and large to be attributed to Stygimoloch, which possesses a smaller, mediolaterally narrow dome ^[8]. However, due to the large frontoparietal dome of Pachycephalosaurus wyomingensis among other pachycephalosaurids, the large size of BMR P2001.4.5 suggests the specimen is referable to Pachycephalosaurus^[16].

Previous studies have called the validity of Stygimoloch and Dracorex into question, suggesting that they are younger ontogenetic stages of P. wyomingensis^[15]. Furthermore, Goodwin and Horner ^[15] have suggested that variation in frontoparietal surface texture may be a factor of ontogeny and dome development. Regardless of this debate, the nearly complete fusion and closure of the frontoparietal suture, the massive size of BMR P2001.4.5, and lack of dorsal surface texturing or sulci suggest that it is a mature individual and thus maintains its identification as Pachycephalosaurus wyomingensis.

Dinosauria Owen, 1842 ^[20]

Ornithischia Seeley, 1887 ^[21]

Pachycephalosauria Maryanska and Osmolska, 1974 ^[22]

Pachycephalosauridae Sternberg, 1945 ^[23]

Pachycephalosaurinae Sereno, 2000 ^[2]

Pachycephalosaurus wyomingensis Brown and Schlaikjer, 1943 ^[16]

A series of small erosive pits and two large crateriform depressions occur on the dorsal surface of BMR P2001.4.5 (Figures 1, 2). The two large depressions are positioned on the frontal region of the dorsal surface, measure approximately 5 cm in diameter, and have a maximum depth of 1.6 cm. The inner surface and marginal rim of the depressions have several smaller circular pits (Figures 1B, 3A, D) ranging from 1 to 10 mm in diameter and depth, the more shallow of which possess smooth surfaces. The larger pits flare where they penetrate into the dome, resulting in a flask-shaped cross section (Figure 3A), and possess horizontally oriented canals that extend into the larger depressions (Figures 1A, 3D). The larger and deeper structures present on BMR P2001.4.5 penetrate the outer-most cortex of the dorsal surface, exposing an irregular area of compact bone immediately above a lower-density band (Figure 3B, C).

Pachycephalosaurid frontoparietal domes possess a variety of histological zones dependent on genus and ontogenetic stage ^[24]. The middle and least dense of these zones, Zone II, occurs in subadult individuals, and increases in density throughout ontogeny. Coring or thin sectioning of BMRP 2001.4.5 was not permitted; however a series of computed tomography (CT) scans were conducted to gain further insight into the ontogenetic stage and depression characteristics within the specimen. The scan was performed with an Aquilion Toshiba 64-Slice CT scanner at Rockford Memorial Hospital in Rockford, IL, and conducted at settings for medical diagnoses of bone pathology (135 kV, 300 mA, 0.5 mm pixel resolution, and 0.5 mm thickness). The significant thickness and very high density of BMR P2001.4.5 caused considerable noise in the CT images, which decreased some of the examination detail. However, this did not prevent examination and characterization of the important structural features of the depressions. The raw CT data are archived at the Burpee Museum of Natural History. The frontoparietal dome was also digitized into a 3D model using a NextEngine Desktop 3D Scanner (Figure S1).

While histologic sections could not be produced for this study, BMR P2001.4.5 was compared with similar osteological structures in fossil and extant archosaurs in order to infer the cause of the depressions and erosive pits. Explored causes include taphonomic alteration (i.e. insect modification and bone weathering), non-traumatic bone resorption, and traumatic origins (i.e. injury and subsequent infection).

Taphonomic alteration resulting from biostratinomic and diagenetic processes is commonly responsible for erosional features in bone ^[25]–^[27]. Indentations and borings in bones due to early biostratinomic processes such as scavenging ^[28] and insect modification ^[26] are common in the vertebrate fossil record. However, such features can usually be attributed to these origins by specific traces such as parallel puncture and drag marks in scavenged bones ^[29] and mandible marks from insect modification ^[26]. The absence of these traces on BMR P2001.4.5 suggests that post-mortem scavenging and insect damage are not likely etiologies.

Previous occurrences of erosive and pitted structures on pachycephalosaurid crania have been regarded as a result of taphonomic weathering and erosion due to subaerial exposure. Tanke and Farke ^[13] noted possible pitting in frontoparietal domes of Stegoceras, but suggested the features might have been due to water-wear. Sullivan ^[30] also commented on the possibility of taphonomic effects as cause for features present in the holotypes of Gravitholus albertus and Prenocephale edmontonensis. The effects of weathering and erosion on both ancient and modern bone have been thoroughly described ^[25], ^[31]–^[33]. Parallel cracks and polygonal fracture patterns commonly characterize subaerially exposed and eroded bone surfaces ^[25], ^[33]. The surface of BMR P2001.4.5 lacks these characteristics, suggesting that the depressions and pits are not likely the result of extensive subaerial exposure and erosion.

Lesions appearing in the squamosal fenestrae of numerous specimens of chasmosaurine ceratopsids have been attributed to bone resorption ^[13]. Bone remodeling or “punched out lesions” (POLs) described in ceratopsians usually occur in thin regions of the squamosal, on both internal and external bone surfaces, and the lesions exhibit smooth surfaces ^[13]. The structures on BMR P2001.4.5 exhibit irregular lesion surfaces that extend into deeper bone tissues, unlike the smooth surfaces in POLs. Furthermore, the massive size of BMR P2001.4.5 is also inconsistent with POLs, such as those commonly seen in thin ceratopsian squamosals and parietals. This suggests that non-traumatic bone resorption is not a likely source of the depressions.

Lesions in bone due to injury and disease have been well documented, and may best explain the origins of the lesions in BMR P2001.4.5. Osteomyelitis is the infection of bone and bone marrow and can be classified as acute, subacute or chronic. Chronic osteomyelitis is a severe, persisting infection of bone and bone marrow, and can be the result of trauma to the bone itself or spread from adjacent soft tissue infection. An anatomic classification of chronic osteomyelitis has been proposed by Cierny and Mader ^[34] (Table 1).

The acute phase of osteomyelitis generally does not show radiographic abnormalities. Findings of acute or subacute osteomyelitis, which is commonly bacterial or fungal in origin, include periosteal reaction, cortical irregularity and demineralization, while findings of chronic osteomyelitis include thick, sclerotic, irregular bone, and an elevated periosteum ^[35].

If a wound is present and shows signs of infection, then any exposed bone in the bed of the wound is considered infected. An open fracture, one in which there is bone exposed to the outside environment, is at increased risk for development of superimposed infection. The amount of overlying soft tissue on pachycephalosaurid domes is unknown but the supposition that it was minimal ^[5] would lead to a higher likelihood of direct contamination. Subsequent ongoing exposure to the surrounding environment would logically provide additional opportunity for contamination.

Open fractures in the human medical literature are classified based on the mechanism of injury, degree of soft tissue damage, fracture configuration and level of contamination ^[36]; the incidence of infection is influenced by the type of open fracture. The lesions present on BMR P2001.4.5 correlate well with the higher level of the classification scheme, Type III – B in which the fracture is associated with prominent injury to (or loss of) soft tissue, periosteal stripping and exposure of bone, and massive contamination ^[37]. Any possible comminuted fracture fragments could have left the fracture site immediately as the result of the force applied or could have fallen from the parent bone in a delayed fashion. Over time, infected bone fragments (i.e. sequestra) can be ejected from a site of chronic osteomyelitis through a cloaca, or resorbed ^[35].

The CT images show chronic changes involving the surface of the concave bone lesion where the margins are smooth and rounded compatible with healing (Figure 3B–E). The expected thick, sclerotic nature of bone tissue seen with chronic osteomyelitis may be masked by the nature of the particular bone anatomy for pachycephalosaurids ^[22]. However, the expected irregularity of the floor of the lesion is well demonstrated in BMR P2001.4.5. These irregular floors show a varying thickness of higher density bone over a rarefied zone which does not exist at the same depth elsewhere in the specimen, consistent with woven osseous remodeling and overlying healing new bone (Figure 3C). A few small rounded concavities penetrate the bone surface at the periphery of the larger defect (Figures 3D, E). Their more focal nature is characterized by acute margins consistent with localized erosions. These characteristics suggest the lesions are of traumatic etiology with superimposed, ongoing infection.