[D] Stegoceras novomexicanum
Describer
Jasinski & Sullivan 2011
Time
Cretaceous Late Campanian
Classification
Ornithischia Genasauria Cerapoda Marginocephalia Pachycephalosauria Pachycephalosauridae
Diet
Herbivore
Fossilsite
Fruitland Formation (Fossil Forest Member) and lower Kirtland Formation (Hunter Wash Member) San Juan Basin, New Mexico, US
Fall Under
Stegoceras
Info
Stegoceras (Lambe, 1902) > Stegoceras validum (Lambe, 1918) >> Stegoceras breve (Lambe, 1918)
Stegoceras > Stegoceras edmontonense (Brown & Schlaikjer, 1943) > Troodon edmontonensis (Brown & Schlaikjer, 1943)
Stegoceras > Ornatotholus (Galton & Sues, 1983) >> Stegoceras browni (Wall & Galton, 1979) >> Ornatotholus browni (Wall & Galton, 1979)
Stegoceras > Stegoceras novomexicanum (Jasinski & Sullivan 2011)
Abstract
Two partial pachycephalosaurid skulls, from the upper Fruitland and lower Kirtland formations (Upper Cretaceous), are recognized as belonging to a new species Stegoceras Lambe, S. novomexicanum, n. sp. Stegoceras novomexicanum differs from the only other recognized species of Stegoceras (sensu Sullivan, 2003) in possessing: a reduced and sub-rectangular posteromedial extension of the parietal; parallel squamosal sutural surface contacts of the posteromedial extension of the arietal; enlarged and medially positioned supratemporal fenestrae; and a small (adult) size. Fusion of the frontal and parietal in one specimen, coupled with a smooth orsal surface of the frontoparietal dome, is consistent with an adult ontogenetic stage. Gross histology reveals four histomorphs, the fourth (outer-most layer) indicates rrested growth, further attesting to its mature state.
Stegoceras novomexicanum is known from, and restricted to, the upper Fruitland Formation (Fossil Forest Member) and lower Kirtland Formation (Hunter Wash Member); the collective vertebrates from these contiguous strata make up the Hunter Wash local fauna. Contrary to previous reports, the Prenocephale (= Sphaerotholus) -like pachycephalosaurids are not known from the early Kirtlandian, but are restricted to the Willow Wash local fauna of the upper Kirtland Formation (De-na-zin Member).
Stegoceras novomexicanum is temporally younger (Kirtlandian) than the well-known S. validum from the Judithian of Alberta, Canada. A reassessment of the newly named taxon Texacephale langstoni demonstrates that it is not based on diagnostic material and, therefore, is a nomen dubium.
Etymology
The species name is derived from the State of New Mexico, from which the holotype and paratypes came.
Diagnosis
Differs from Stegoceras validum (the holotype, CMN 515, Fig. 3D-F) in possessing the following features: posteromedial extension of the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal roughly parallel; supratemporal fenestrae more medial and enlarged; gracile and small adult size.
Holotype
NMMNH P-33898, nearly complete frontoparietal.
Paratypes
SMP VP-2555, greater posterior part of left frontal and anterior-most portions of left and right frontals; SMP VP-2790, incomplete parietal.
Type Locality
NMMNH locality 4716, San Juan Basin, New Mexico. Precise coordinates retained at the NMMNH and are available to qualified researchers.
Paratype Locality
SMP VP-2555 is from the upper Fruitland Formation (Fossil Forest Member), late Campanian (early Kirtlandian) Member), late Campanian (early Kirtlandian); SMP VP-2970 is from the lower part of the Kirtland Formation (Hunter Wash Member), late Campanian (early Kirtlandian) at SMP locality 461 (“Target 9”), San Juan Basin, New Mexico.
Formation/Age
Upper Fruitland Formation (Fossil Forest Member) to lower Kirtland Formation (Hunter Wash Member), late Campanian (early Kirtlandian).
Emended Description
The description presented by Sullivan and Lucas is emended by the following observations: the posteromedial extension of the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal parallel; supratemporal fenestrae are large and relatively close together; and small size.
Remarks
The features that serve to distinguish Stegoceras novomexicanum from Stegoceras validum (posteromedial extension of the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal parallel; supratemporal fenestrae more medial and enlarged; and small adult size) are either present or inferred to be present in the two paratype specimens (SMP VP-2555 and VP-2790) based on cross comparisons with the holotype.
We note here that the anterior margins of the supratemporal fenestrae are intact in the holotype Stegoceras novomexicanum (NMMNH P-33898) and the general shape of the posteromedial extension of the parietal is narrow compared to the holotype of Stegoceras validum (CNM 515). While it could be argued that the holotype of S. novomexicanum represents an immature individual, we believe that it is mature and it is taxonomically distinct from Stegoceras validum. When considering other features that are present in the referred material, it is evident that we are dealing with a species that is nearly full grown based on: 1) smoothness of the frontoparietal dome (in SMP VP-2555, a slightly larger and more mature individual); 2) a capping histological layer in both SMP VP-2555 and SMP VP-2790, suggesting arrested growth; and 3) partial fusion of the frontals, as well as the frontoparietal, as seen in SMP VP-2555. These observations are discussed in greater detail below.
In addition, when considering that the New Mexico specimens are biostratgraphically younger and geographically separate from Stegoceras validum these additional facts would further support the recognition of a separate species.
Jasinski & Sullivan 2011
Time
Cretaceous Late Campanian
Classification
Ornithischia Genasauria Cerapoda Marginocephalia Pachycephalosauria Pachycephalosauridae
Diet
Herbivore
Fossilsite
Fruitland Formation (Fossil Forest Member) and lower Kirtland Formation (Hunter Wash Member) San Juan Basin, New Mexico, US
Fall Under
Stegoceras
Info
Stegoceras (Lambe, 1902) > Stegoceras validum (Lambe, 1918) >> Stegoceras breve (Lambe, 1918)
Stegoceras > Stegoceras edmontonense (Brown & Schlaikjer, 1943) > Troodon edmontonensis (Brown & Schlaikjer, 1943)
Stegoceras > Ornatotholus (Galton & Sues, 1983) >> Stegoceras browni (Wall & Galton, 1979) >> Ornatotholus browni (Wall & Galton, 1979)
Stegoceras > Stegoceras novomexicanum (Jasinski & Sullivan 2011)
Abstract
Two partial pachycephalosaurid skulls, from the upper Fruitland and lower Kirtland formations (Upper Cretaceous), are recognized as belonging to a new species Stegoceras Lambe, S. novomexicanum, n. sp. Stegoceras novomexicanum differs from the only other recognized species of Stegoceras (sensu Sullivan, 2003) in possessing: a reduced and sub-rectangular posteromedial extension of the parietal; parallel squamosal sutural surface contacts of the posteromedial extension of the arietal; enlarged and medially positioned supratemporal fenestrae; and a small (adult) size. Fusion of the frontal and parietal in one specimen, coupled with a smooth orsal surface of the frontoparietal dome, is consistent with an adult ontogenetic stage. Gross histology reveals four histomorphs, the fourth (outer-most layer) indicates rrested growth, further attesting to its mature state.
Stegoceras novomexicanum is known from, and restricted to, the upper Fruitland Formation (Fossil Forest Member) and lower Kirtland Formation (Hunter Wash Member); the collective vertebrates from these contiguous strata make up the Hunter Wash local fauna. Contrary to previous reports, the Prenocephale (= Sphaerotholus) -like pachycephalosaurids are not known from the early Kirtlandian, but are restricted to the Willow Wash local fauna of the upper Kirtland Formation (De-na-zin Member).
Stegoceras novomexicanum is temporally younger (Kirtlandian) than the well-known S. validum from the Judithian of Alberta, Canada. A reassessment of the newly named taxon Texacephale langstoni demonstrates that it is not based on diagnostic material and, therefore, is a nomen dubium.
Etymology
The species name is derived from the State of New Mexico, from which the holotype and paratypes came.
Diagnosis
Differs from Stegoceras validum (the holotype, CMN 515, Fig. 3D-F) in possessing the following features: posteromedial extension of the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal roughly parallel; supratemporal fenestrae more medial and enlarged; gracile and small adult size.
Holotype
NMMNH P-33898, nearly complete frontoparietal.
Paratypes
SMP VP-2555, greater posterior part of left frontal and anterior-most portions of left and right frontals; SMP VP-2790, incomplete parietal.
Type Locality
NMMNH locality 4716, San Juan Basin, New Mexico. Precise coordinates retained at the NMMNH and are available to qualified researchers.
Paratype Locality
SMP VP-2555 is from the upper Fruitland Formation (Fossil Forest Member), late Campanian (early Kirtlandian) Member), late Campanian (early Kirtlandian); SMP VP-2970 is from the lower part of the Kirtland Formation (Hunter Wash Member), late Campanian (early Kirtlandian) at SMP locality 461 (“Target 9”), San Juan Basin, New Mexico.
Formation/Age
Upper Fruitland Formation (Fossil Forest Member) to lower Kirtland Formation (Hunter Wash Member), late Campanian (early Kirtlandian).
Emended Description
The description presented by Sullivan and Lucas is emended by the following observations: the posteromedial extension of the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal parallel; supratemporal fenestrae are large and relatively close together; and small size.
Remarks
The features that serve to distinguish Stegoceras novomexicanum from Stegoceras validum (posteromedial extension of the parietal reduced and sub-rectangular; squamosal sutural surface contacts of the posteromedial extension of the parietal parallel; supratemporal fenestrae more medial and enlarged; and small adult size) are either present or inferred to be present in the two paratype specimens (SMP VP-2555 and VP-2790) based on cross comparisons with the holotype.
We note here that the anterior margins of the supratemporal fenestrae are intact in the holotype Stegoceras novomexicanum (NMMNH P-33898) and the general shape of the posteromedial extension of the parietal is narrow compared to the holotype of Stegoceras validum (CNM 515). While it could be argued that the holotype of S. novomexicanum represents an immature individual, we believe that it is mature and it is taxonomically distinct from Stegoceras validum. When considering other features that are present in the referred material, it is evident that we are dealing with a species that is nearly full grown based on: 1) smoothness of the frontoparietal dome (in SMP VP-2555, a slightly larger and more mature individual); 2) a capping histological layer in both SMP VP-2555 and SMP VP-2790, suggesting arrested growth; and 3) partial fusion of the frontals, as well as the frontoparietal, as seen in SMP VP-2555. These observations are discussed in greater detail below.
In addition, when considering that the New Mexico specimens are biostratgraphically younger and geographically separate from Stegoceras validum these additional facts would further support the recognition of a separate species.