Craniofacial ontogeny in Tylosaurinae.

Area of Science:

• Paleontology
• Vertebrate Zoology
• Evolutionary Biology

Background:

• Mosasaurs were dominant marine reptiles during the Late Cretaceous, with extensive fossil records.
• Despite numerous specimens, detailed ontogenetic (growth) series for mosasaurs remain largely unestablished.
• The genus Tylosaurus, known for its large size and distinct cranial features, offers potential for growth studies.

Purpose of the Study:

• To establish detailed growth series for Tylosaurus proriger and the Tylosaurus kansasensis/nepaeolicus complex.
• To test hypotheses regarding the synonymy of T. kansasensis and T. nepaeolicus, and investigate sexual dimorphism, anagenesis, and heterochrony within Tylosaurus.

Main Methods:

• Identification of 59 hypothetical growth characters (size-dependent, size-independent, phylogenetic).
• Quantitative cladistic analysis to reconstruct growth series for the studied Tylosaurus taxa.
• Spearman rank-order correlation to assess the relationship between size metrics and maturity.

Main Results:

• The study supports the synonymy of Tylosaurus kansasensis with Tylosaurus nepaeolicus, with T. kansasensis representing juvenile forms.
• Significant correlations were found between skull length, quadrate height, and maturity.
• Eleven shared growth changes were identified across both species; no evidence of skeletal sexual dimorphism was found, and a hypothesis of paedomorphy in T. proriger was not rejected.

Conclusions:

• The findings clarify the ontogeny of Tylosaurus, integrating T. kansasensis into the T. nepaeolicus growth series.
• Growth in Tylosaurus is strongly size-dependent, lacking discernible sexual dimorphism in the studied specimens.
• A new hypothesis of anagenesis driven by peramorphy is proposed for Western Interior Seaway Tylosaurus species.