Fossoriality and evolutionary development in two Cretaceous mammaliamorphs

Area of Science:

• Paleontology
• Vertebrate Evolution
• Developmental Biology

Background:

• Mammaliamorpha includes the ancestors and descendants of Tritylodontidae and Mammalia.
• Tritylodontids were herbivorous cynodonts from the Late Triassic to Early Cretaceous.
• Eutriconodontans are considered an extinct mammalian group with varying classifications.

Purpose of the Study:

• To report the discovery of a new tritylodontid and eutriconodontan from the Early Cretaceous Jehol Biota.
• To investigate convergent adaptations for fossorial (digging) life in these distantly related species.
• To analyze evolutionary changes in the axial skeleton and their developmental underpinnings in stem mammaliamorphs.

Main Methods:

• Fossil discovery and analysis within the Jehol Biota.
• Comparative morphology to identify convergent features and skeletal adaptations.
• Examination of vertebral counts and characteristics to infer developmental patterns.

Main Results:

• Discovery of the first tritylodontids in the Jehol Biota, alongside common eutriconodontans.
• Both species exhibit convergent adaptations for fossorial life, identified as 'scratch-diggers'.
• An increased number of presacral vertebrae with meristic and homeotic changes were observed, indicating evolutionary shifts.

Conclusions:

• These fossils provide insights into the evolutionary development of the mammaliamorph axial skeleton.
• The findings suggest that developmental plasticity in somitogenesis and HOX gene expression was present in stem mammaliamorphs.
• The interplay of developmental mechanisms and natural selection likely drove diverse body plan evolution in mammaliamorph clades.