Lecture Notes: Cranial Anatomy of Allosaurus jimmadseni

Introduction

Allosaurus is the most common theropod dinosaur in the Late Jurassic of North America, widely found in various quarries.
Despite its abundance, well-preserved complete skeletons are rare, mostly represented by scattered elements, especially in the Cleveland-Lloyd Dinosaur Quarry.
Allosaurus plays a crucial role in phylogenetic analyses of Theropoda.
Nineteen species have been proposed, but many are not well-illustrated or described, leading to ongoing debates about their validity.
Current recognition includes three species: Allosaurus fragilis, Allosaurus jimmadseni in North America, and Allosaurus europaeus in Europe.

Discovery and Excavational History

Two key specimens: DINO 11541 from Utah and MOR 693 from Wyoming.
DINO 11541 discovered in 1990, excavation involved innovative methods due to difficult terrain.
MOR 693, found articulated at Howe Quarry in the 1990s, led to the discovery of "Big Al".
Allosaurus jimmadseni is proposed as a new species based on these specimens, differentiating it from other known species based on cranial features.

Materials and Methods

Specimens DINO 11541 and MOR 693 are reposited in public repositories, collected under proper permits.
Comparisons made with a wide range of theropod taxa and literature on non-coelurosaurian theropods.

Systematic Paleontology

Allosaurus jimmadseni is distinct from other species based on a unique combination of cranial features.
Holotype is DINO 11541 with nearly complete skeleton; MOR 693 also a key specimen.

Description and Comparisons of Allosaurus jimmadseni

Skull and Lower Jaw: Found articulated, exhibits typical theropod features with some unique characteristics.
Cranial Fenestrae and Fossae: Detailed examination of external narial fossae, narial fossa, antorbital fossae, etc.
Bones of the Dermatocranium: Distinct features in premaxillae, maxillae, nasals, lacrimals, frontals, and others highlighted.
Bones of the Splanchnocranium: Includes hyoid and stapes features.
Dentition: Features five premaxillary teeth, unique among allosauroids.

Ontogenetic Assessment

DINO 11541 is one of the most complete theropod skeletons, providing valuable comparative data.
Not fully mature at the time of death, suggesting potential for larger size.

Conclusions

Allosaurus jimmadseni and Allosaurus fragilis are valid distinct species in the Morrison Formation.
Unique cranial features distinguish Allosaurus jimmadseni from other species, aiding in phylogenetic studies.

Supplemental Information

Authors declare no competing interests.
Data available in the article, specimens accessible in supplemental files.
New species registered with ZooBank.

Funding

Supported by Dinosaur National Monument, National Parks Service, and other institutions.

These notes provide an overview of the significant findings and discussions from the lecture on the cranial anatomy of Allosaurus jimmadseni and its distinction from other species.