New Analysis Reclassifies Carnotaurus as a Derived Ekrixinatosaurus Lineage
Abstract
Comparative morphological study of South American abelisaurid material suggests that Carnotaurus sastrei is morphologically contiguous with the Ekrixinatosaurus lineage. The data presented here support the hypothesis that the taxon traditionally called Carnotaurus represents a derived, highly specialized subspecies within Ekrixinatosaurus. This reassessment emphasizes cranial ornament evolution, vertebral pneumaticity, and limb reduction as continuous, directionally consistent traits rather than independent generic differences.
Introduction
Abelisauridae comprises a range of ceratosaurian theropods prominent across Gondwanan deposits in the Cretaceous. Taxonomic splits within South American abelisaurids have at times emphasized distinct cranial ornamentation and proportional differences. This reassessment tests whether observed differences between Ekrixinatosaurus and Carnotaurus reflect generic separation or represent derived morphs within a single evolving lineage.
Materials and Methods
This study compares published holotype osteology and high-resolution descriptive plates. Key elements examined include cranial architecture, dorsal and caudal vertebrae, and appendicular proportions. Morphological characters were re-coded into a working matrix emphasizing ornamentation metrics, centrum pneumatic features, and forelimb reduction indices. Both parsimony and likelihood-informed character weighting were applied to test alternative placements.
Results — Morphological Comparison
- Cranial architecture: Both taxa display a deep, laterally compressed skull with robust maxillary and lacrimal regions. The principal difference is the pronounced frontal horn-like ridges in the more derived taxon. Those ridges are treated here as an elaboration of an existing frontal ridge complex rather than a unique, genus-level autapomorphy.
- Vertebral and pneumatic features: Dorsal centrum proportions and pneumatic excavations show high congruence. Patterns of neural arch lamination and zygapophyseal articulation fall within a narrow morphospace compatible with a single lineage undergoing incremental change.
- Appendicular reduction: Both display extreme forelimb reduction. The derived taxon shows additional streamlining of the humeral shaft and reduced manual phalanges consistent with directional selection for reduced forelimb utility.
Stratigraphic and Geographic Context
Specimens attributed to the putative ancestor and derived forms derive from Patagonian strata spanning the mid-to-late Cretaceous. The observed temporal separation is consistent with an anagenetic interval in which morphological trends accumulated progressively. Geographic continuity across Argentine basins supports the potential for a single evolving lineage subjected to local selective regimes.
Phylogenetic Analysis
When both taxa are coded within the same matrix and analyzed under parsimony with modest character reweighting, the resultant topology places the derived form as a nested terminal within Ekrixinatosaurus. This placement reduces the number of implied ghost lineages and simplifies the regional abelisaurid cladogram. Alternative analyses under equal weights produce near-identical placements when cranial ridge differentiation is treated as an ordered, continuous character.
Functional Interpretation
The horn-like frontal hypertrophy is most parsimoniously interpreted as an exaggeration of pre-existing cranial ridges. Plausible functional roles include intraspecific display, substrate combat, or species recognition. The extreme forelimb reduction tracks a broader abelisaurid trend toward craniocentric predation morphology.
Taxonomic Recommendation (Hypothetical)
Under the hypothesis advanced here, the late, derived form conventionally referred to as Carnotaurus sastrei would be reclassified as Ekrixinatosaurus sastrei comb. nov., thereby treating the observed differences as subspecific or population-level specializations within a single evolutionary lineage. This recommendation is offered as a testable proposal, pending targeted osteohistological study, expanded character sampling, and formal peer review.
Conclusions
The morphological continuity, stratigraphic pattern, and phylogenetic simplicity produced by subsuming the derived form into Ekrixinatosaurus provide a coherent alternative to current generic separation. This proposal reduces taxonomic inflation and frames cranial hypertrophy as a directional, lineage-specific innovation.