Project 4097: M. R. Whitney, K. D. Angielczyk, B. R. Peecook, C. A. Sidor. 2021. The evolution of the synapsid tusk: insights from dicynodont therapsid tusk histology. Proceedings of the Royal Society B: Biological Sciences. 288 (1961):null.
Specimen: † Diictodon
View: transverse


The mammalian tusk is a unique and extreme morphotype among modern vertebrate dentitions. Tusks—defined here as ever-growing incisors or canines composed of dentine—evolved independently multiple times within mammals yet have not evolved in other extant vertebrates. This suggests that there is a feature specific to mammals that facilitates the evolution of this specialized dentition. To investigate what may underpin the evolution of tusks, we histologically sampled the tusks of dicynodont therapsids: the earliest iteration of tusk evolution and the only non-mammalian synapsid clade to have acquired such a dentition. We studied the tissue composition, attachment tissues, development and replacement in 10 dicynodont taxa and show multiple developmental pathways for the adult dentitions of dicynodont tusks and tusk-like caniniforms. In a phylogenetic context, these developmental pathways reveal an evolutionary scenario for the acquisition of an ever-growing tusk—an event that occurred convergently, but only in derived members of our sample. We propose that the evolution of an ever-growing dentition, such as a tusk, is predicated on the evolution of significantly reduced tooth replacement and a permanent soft-tissue attachment. Both of these features are fixed in the dentitions of crown-group mammals, which helps to explain why tusks are restricted to this clade among extant vertebrates.

Read the article »

Article DOI: 10.1098/rspb.2021.1670

Project DOI: 10.7934/P4097,
This project contains
  • 16 Media
  • 9 Taxa
  • 12 Specimens
Total size of project's media files: 410.88M

Download Project SDD File
Currently Viewing:
MorphoBank Project 4097

    This research
    supported by

    Authors' Institutions

    • Harvard University

    • Idaho State University

    • University of Washington

    • Field Museum of Natural History


    member name taxa specimens media
    Megan Whitney
    Project Administrator

    Project has no matrices defined.

    Project views

    type number of views Individual items viewed (where applicable)
    Total project views4171
    Project overview253
    Media views1731Media search (468 views); M823721 (82 views); M823724 (78 views); M823723 (79 views); M823730 (79 views); M823728 (84 views); M823731 (78 views); M823735 (76 views); M823733 (80 views); M823734 (80 views); M823736 (75 views); M823726 (74 views); M823732 (83 views); M823727 (79 views); M823729 (79 views); M823725 (78 views); M823722 (79 views);
    Views for media list287
    Specimen list1248
    Taxon list552

    Project downloads

    type number of downloads Individual items downloaded (where applicable)
    Total downloads from project38
    Project downloads36
    Media downloads2M823731 (1 download); M823727 (1 download);