Comparative cranial ontogeny of Tapirus (Mammalia: Perissodactyla: Tapiridae)

Abstract

Skull morphology in tapirs is particularly interesting due to the presence of a proboscis with important trophic, sensory and behavioral functions. Several studies have dealt with tapir skull osteology but chiefly in a comparative framework between fossil and recent species of tapirs. Only one study examined an aspect of cranial ontogeny, development of the sagittal crest (Holbrook. J Zool Soc Lond 2002; 256; 215). Our goal is to describe in detail the morphological changes that occur during the postnatal ontogeny of the skull in two representative tapir species, Tapirus terrestris and Tapirus indicus, and to explore possible functional consequences of their developmental trajectories. We compared qualitative features of the skull on a growth series of 46 specimens of T. terrestris ordered on the basis of the sequence of eruption and tooth wear, dividing the sample into three age classes: class Y (very young juvenile), class J (from young juvenile to young adult) and class A (full and old adult). The qualitative morphological analysis consisted of describing changes in the series in each skull bone and major skull structure, including the type and degree of transformation (e.g. appearance, fusion) of cranial features (e.g. processes, foramina) and articulations (sutures, synchondroses, and synovial joints). We then measured 23 cranial variables in 46 specimens of T. terrestris that included the entire ontogenetic series from newborn to old adults. We applied statistical multivariate techniques to describe allometric growth, and compared the results with the allometric trends calculated for a sample of 25 specimens of T. indicus. Results show that the skull structure was largely conserved throughout the postnatal ontogeny in T. terrestris, so class Y was remarkably similar to class A in overall shape, with the most significant changes localized in the masticatory apparatus, specifically the maxillary tuber as a support of the large-sized permanent postcanine dentition, and correlated changes in diastemata, mandibular body, and sagittal and nuchal crests. In the nasal region, ontogenetic remodeling affected the space for the meatal diverticulum and the surfaces for the origin of the proboscis musculature. Overall, ontogenetic trajectories exhibited more negative allometric components in T. indicus than in T. terrestris, and they shared 47.83% of allometric trends. Tapirus indicus differed most significantly from T. terrestris in the allometry of postcanine toothrows, diastemata and mandibular body. Thus, some allometric trends seem to be highly conserved among the species studied, and the changes observed showed a strong functional and likely adaptive basis in this lineage of ungulates.

Clade‐specific evolutionary diversification along ontogenetic major axes in avian limb skeleton

“…Still, there are empirical data indicating that certain aspects of postnatal ontogenetic trajectories are conserved across closely related taxa (e.g., Larson ; Scott et al. ; Moyano and Giannini ). As pointed out by Cheverud and Marroig () regarding comparison of covariance matrices, a statistically significant difference does not necessarily represent a biologically meaningful one.…”

Cranial Morphology and Phylogenetic Relationships of Trigonostylops wortmani, an Eocene South American Native Ungulate

“…A different feature, equally unusual, is the meatal diverticulum seen in tapirs. This is an epitheliumlined saclike extension of the nasal cavity that extends onto each side of the upper face of tapirs; its position is marked on the skull by bony gutters running along the external margin of the nasal aperture (Witmer et al, 1999;Moyano and Giannini, 2017). Somewhat similar features occur in an even more elaborate form in certain macraucheniids (Forasiepi et al, 2016), but have not been identified as such in any other SANUs, including trigonostylopids and astrapotheriids.…”

“…23; cf. Moyano and Giannini, 2017). In tapirs each facial process is robust and bears a full incisor dentition, as in the horse, but in rhinos the process is small, bearing either one incisor or none at all (Nowak, 1999).…”

“…By contrast, in all extant perissodactylans the lacrimal has a distinct facial component. In tapirs and rhinos, double lacrimal foramina commonly occur on the orbital rim (not within the orbit); Equus has a single foramen, also within the orbit (Sisson and Grossman, 1953;O'Leary and Gatesy, 2008;Moyano and Giannini, 2017).…”

“…indicus, although given that the cartilage of the auditory tube would have differentiated in the same area this cannot be regarded as conclusive. Moyano and Giannini (2017) did not specifically discuss the entotympanic, but as their study is based on numerous specimens it is likely that a separate element was never encountered in the tympanic region. For the present entotympanic presence in tapirs must remain undetermined.…”