Data from: Palaeohistology and life history of the early Palaeocene taeniodont Conoryctes comma (Mammalia: Eutheria)

The life histories of Palaeocene mammals are poorly known, but may have
been central to their success in diversifying across terrestrial
ecosystems after the end-Cretaceous extinction. Among these mammalian
groups, the eutherian Taeniodonta are particularly enigmatic, with few
modern analogues and no living descendants, despite being one of the only
lineages to apparently traverse the Cretaceous-Palaeogene (K-Pg) boundary.
Here we investigate the life history of an early Palaeocene taeniodont,
Conoryctes comma, based on a multi-individual, multi-element sample. We
produced palaeohistological thin sections of skeletal elements, including
one tooth and 26 postcranial bones, to produce 36 slides. Nearly all
elements sampled exhibit similar osteohistological architecture, with a
small internal zone of compacted coarse cancellous bone surrounded by an
internal cortex of periosteally-derived fibrolamellar bone of variable
thickness, and an outer cortex of lamellar bone. The well-vascularized
fibrolamellar complex in the limb bones, lacking cyclical growth marks, is
indicative of overall rapid growth to near adult body size. Cyclical
growth marks are present in the outer cortex after the transition to
slow-growing lamellar bone, but not in the inner cortex, suggesting sexual
maturity was reached in one year. In some elements, an internal growth
mark shares histological similarities with weaning marks in living mammals
and other contemporary Palaeocene mammals, and occurred at the body size
predicted for this transition in therian mammals. The unusual presence of
compacted coarse cancellous bone near the midshafts of multiple limb bones
may be related to cortical thickening, and is similar to the arrangement
described in some fossorial mammals, supporting previous assertions of
this lifestyle in Conoryctes. Altogether, these palaeohistological signals
suggest a life history in Conoryctes comma similar to living eutherians,
despite uncertainty about whether it is within crown Placentalia or a
close outgroup. Thus, our data are consistent with an early origin of
placental-like reproductive strategies in their eutherian ancestors,
although this attribute was likely shared more broadly among Mesozoic
lineages prior to the end-Cretaceous extinction. These data may be useful
in the future for broader evaluations of early Palaeocene mammal growth
and life history, or for other physiological proxies.