Data, code, and supplementary information for Hyracoid locus identification

Serially homologous elements pose an identification problem in fragmentary
records, particularly those of vertebrate fossils. Examples include
individual vertebrae in the vertebral column and teeth in a tooth row.
Until an isolated element can be accurately attributed to a specific
position within its series, multiple lines of ecological and evolutionary
research cannot be conducted. However, varying levels of differentiability
between loci, and varying patterns of differentiation across clades, make
it impossible to develop a single set of diagnostic traits for any
particular set of serial homologs, particularly mammalian molars. Here, we
test the utility of a set of classification criteria for distinguishing
molar tooth positions of hyraxes (Mammalia, Afrotheria, Hyracoidea), which
have been considered indistinguishable in previous taxonomic studies. As
part of the test, we evaluate the degree to which between-locus variation
is conservative in this taxon, which would strengthen the predictive power
of proposed traits even in cases where species identity is unknown.
Suitable tests for hypotheses of conservatism in categorical traits did
not exist, to our knowledge, and we, therefore, explored the behavior of
previously developed metrics, Borges et al.’s δ, to assess conservatism in
contrast to the phylogenetic signal produced by Brownian motion. This
metric shows some promise but the nature of resulting distributions makes
tests difficult to interpret, indicating a line of potential future
methods improvement. We used a linear morphometric characterization of
shape to validate the candidate traits. In the case of hyracoid molars,
relatively simple ratios of linear measurements have strong discriminatory
power despite evolutionary variation in between-locus differences.
Overall, new or understudied taxa are likely to have lower molar loci
differentiable by their relative length and talonid vs. trigonid width.