Rare osteohistological evidence of skeletal maturity in the early diverging traversodontid Scalenodon angustifrons, with comments on histological sampling coverage in Cynodontia

ABSTRACT

To date, 25 species of non-mammalian cynodonts have been histologically sampled, but few record a definitive and well-developed external fundamental system (EFS) indicative of skeletal maturity. Here, I report an EFS in the femoral histology of Scalenodon angustifrons and discuss the factors that could explain why skeletally mature tissue is rarely documented in cynodont osteohistology. My sample includes two equivalently sized individuals that document marked histological variation, as only one shows evidence of slowing growth rate and an EFS. Variation in the bone histology between femora of the same size suggests that attainment of skeletal maturity does not follow a discrete developmental pattern for S. angustifrons, a pattern that is also seen in the bone histology of other traversodontids and some non-gomphodontian cynodonts. From these results, a flexible growth strategy is inferred for S. angustifrons. To contextualize the validity of this pattern among other cynodont species, I summarize the sampling coverage of cynodonts that have been thin-sectioned and report limited coverage across size classes/ inferred development stages. This is often a result of sampling one isolated element that is not standardized to one skeletal element, which contributes to a poorly constrained comparative sample (in terms of size coverage, spatial and temporal resolution, and taxonomic coverage). Despite this, many cynodont species show shifts towards slow growth, but very few instances of unequivocal evidence of skeletal maturity. This result underscores the need for standardized iterative histological sampling to assess longevity, life history, and developmental patterns of non-mammalian cynodonts more accurately.

ACKNOWLEDGMENTS

I thank the 2007 and 2012 field crews to Tanzania (K. Angielczyk, L. Nampunju, S. Nesbitt, C. Sidor, R. Smith, S. Steyer, M. Stocker, N. Tabor, A. Tibaijuka, L. Tsuji), C. Abramson for preparing NMT RB581, and K. Abrams for her expertise in molding, casting, and specimen preparation. Thanks to L. Marilao for her skilled thin-section preparation, to S. Powers and J. Ricks for their help in curating the Tanzanian collection housed at the Burke Museum, and to Crystal Shin for her expert scientific illustration. For their helpful feedback, edits, and advice, I thank M. Whitney, B. Gee, T. Popowics, A. Huttenlocker, and C. Sidor. Lastly, two anonymous reviewers are thanked for their comments and suggestions that greatly improved the quality of this work. Fossils were collected with support from NGS 7787-05 and NGS 8962-11 (to C. Sidor), NSF DBI-0306158 (to K. Angielczyk).

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author.

DATA AVAILABILITY STATEMENT

The osteohistological data that support the findings of this study are openly available in MorphoBank at http://morphobank.org/permalink/?P4541, reference number P4541.