Romundina and the evolutionary origin of teeth

Area of Science:

• Paleontology
• Evolutionary Biology
• Vertebrate Zoology

Background:

• Theories on vertebrate tooth origins often cite chondrichthyans, but extinct placoderms provide a more relevant evolutionary perspective.
• Placoderms represent a sister clade or grade to living jawed vertebrates (gnathostomes).

Purpose of the Study:

• To investigate the tooth structure and development in the acanthothoracid placoderm Romundina stellina.
• To compare the dentition of Romundina stellina with other placoderms, such as arthrodires, to understand early tooth evolution in jawed vertebrates.

Main Methods:

• Microscopic examination of fossilized 'supragnathal' toothplates from Romundina stellina.
• Comparative analysis of toothplate morphology and histology between different placoderm groups.

Main Results:

• Romundina stellina possesses multi-cuspid teeth with both enameloid caps and dentine cores, added sequentially around a pioneer tooth.
• These teeth are integrated into a continuously growing bony plate.
• Arthrodire placoderms show a similar tooth arrangement but lack enameloid, suggesting its loss in some lineages.
• The dentition appears to have been developmentally distinct from the jaws in early jawed vertebrates.

Conclusions:

• The presence of enameloid in Romundina suggests its secondary loss in other placoderms.
• Covariation of enameloid in teeth and dermal armor implies limited independence in early vertebrate evolution.
• Early vertebrate dentition was likely a discrete developmental unit separate from jaw formation.