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Abstract Although Calligenethlon watsoni, an enigmatic embolomere from Joggins, Nova Scotia, has been known since 1934, an in-depth description of its anatomy (including CT data) and updated taxonomic diagnosis has yet to be completed. Additionally, subsequently discovered embolomere specimens have been referred to the taxon simply if they display embolomere traits. As a result, Calligenethlon is represented by a collection of specimens united on the basis of largely non-diagnostic traits. Here, the exquisitely preserved specimen identified as Calligenethlon, NSM 994GF1.1, is described in detail using micro-computed tomography. Comparison of this specimen to the holotype supports its referral to Calligenethlon and the anatomical knowledge gained here allows for the generation of a more robust morphological diagnosis of the genus Calligenethlon. We then re-evaluate all other referred specimens to determine which are consistent with their referral to the taxon, and which are inconclusively referred to the taxon. These data are discussed as they provide new insights into the anatomy of Calligenethlon and the diversity of embolomeres at the Joggins’ locality. 

The field of ichnology has been a branch of paleontology since the mid 19th century. Vertebrate tracks are a vast dataset compared to vertebrate body fossils: an individual organism will leave only one body, but will leave many tracks and only one body to potentially be preserved. Tracks are direct records of organismal behavior and can be used as proxies for biodiversity and paleoecology. The use of anatomy-consistent morphology in ichnotaxonomy incorporates trackmaker identity by correlating the morphology of the track with that of available track-maker(s). Standard practice is to interpret ichnotaxa to a family level for trackmakers, but the Linnaean rank and cladistic status of trackmaker taxa actually varies greatly. Our work aims to harmonize the ichnological and body fossil records of early Carboniferous tetrapods to better ground trackmaker inferences. For example, the Pennsylvanian-Permian ichnogenus Limnopus is referred to eryopid temnospondyls. However, large ichnospecies (e.g., L. littoralis, L. waynesburgensis) are 200% larger than small ichnospecies (e.g., L. vagus). It has been suggested that the different ichnospecies of Limnopus represent distinct ontogenetic stages of the trackmaker, including the similar but smaller ichnogenus Batrachichnus. Of the few Carboniferous temnospondyls for which the manus and pes are both known in detail, the morphology of the large Limnopus morph is consistent with that of Eryops, but edopid trackmakers cannot be discounted. This has minimal implications for reconstructing ecosystems, given the similarities between eryopids and edopids. However, this uncertainty in the identity of the trackmaker makes the use of Limnopus as a biostratigraphic appearance datum for Eryopidae problematic. The recent consolidation of Limnopus species into the type (L. heterodactylopus) ensures greater ichnotaxonomic consistency, but weakens the track-trackmaker link. Interpretation of Carboniferous pentadactyl tracks must account for the presence of pentadactyly among multiple stem tetrapod families by the late Mississippian. Trackways and skeletal remains — specifically autopodia — are scant in the earlier Mississippian, but both hint at a reduction of pedal digit number to five by this time. The timing of manual digit reduction remains uncertain, but coexistence of a probably hexadactyl manus (Pederpes, Whatcheeriidae) with pendatactyl and tetradactyl manual prints highlights the earliest Carboniferous as a time of diversity and dynamism in early tetrapod morphology.