Tooth Eruption Sequences in Cervids and the Effect of Morphology, Life History, and Phylogeny

Unexpected evolutionary patterns of dental ontogenetic traits in cetartiodactyl mammals

Studying ontogeny in both extant and extinct species can unravel the mechanisms underlying mammal diversification and specialization. Among mammalian clades, Cetartiodactyla encompass species with a wide range of adaptations, and ontogenetic evidence could clarify longstanding debates on the origins of modern specialized families. Here, we study the evolution of dental eruption patterns in early diverging cetartiodactyls to assess the ecological and biological significance of this character and shed new light on phylogenetic issues. After investigation of the ontogenetic dental series of 63 extinct genera, our parsimony reconstructions of eruption state evolution suggest that the eruption of molars before permanent premolars represents a plesiomorphic condition within Cetartiodactyla. This result substantially differs from a previous study based on modern species only. As a result, the presence of this pattern in most ruminants might represent an ancestral condition contributing to their specialized herbivory, rather than an original adaptation. In contrast, the late eruption of molars in hippopotamoids is more likely related to biological aspects, such as increases in body mass and slower pace of life. Our study mainly shows that eruption sequences reliably characterize higher level cetartiodactyl taxa and could represent a new source of phylogenetic characters, especially to disentangle the origin of hippopotamoids and cetaceans.

The relationship between the phosphate and structural carbonate fractionation of fallow deer bioapatite in tooth enamel

RATIONALE

The species-specific relationship between phosphate (δ¹⁸ O_P values) and structural carbonate (δ¹⁸ O_C values) oxygen isotope ratios has been established for several modern and fossil animal species but until now it has not been investigated in European fallow deer (Dama dama dama). This study describes the relationship between phosphate and structural carbonate bioapatite in tooth enamel of extant fallow deer, which will help us further understand the species' unique environmental and cultural history.

METHODS

The oxygen isotope composition of phosphate (δ¹⁸ O_P value) and structural carbonate (δ¹⁸ O_C value) of hydroxylapatite was determined in 51 modern fallow deer tooth enamel samples from across Europe and West Asia. The δ¹⁸ O_C values were measured on a GV IsoPrime dual-inlet mass spectrometer and the δ¹⁸ O_P values on a temperature-controlled elemental analyser (TC/EA) coupled to a DeltaPlus XL isotope ratio mass spectrometer via a ConFlo III interface.

RESULTS

This study establishes a direct and linear relationship between the δ¹⁸ O_C and δ¹⁸ O_P values from fallow deer tooth enamel (δ¹⁸ O_C  = +9.244(±0.216) + 0.958 * δ¹⁸ O_P (±0.013)). Despite the successful regression, the variation in δ¹⁸ O values from samples collected in the same geographical area is greater than expected, although the results cluster in broad climatic groupings when Koppen-Geiger classifications are taken into account for the individuals' locations.

CONCLUSIONS

This is the first comprehensive study of the relationship between ionic forms of oxygen (phosphate oxygen and structural carbonate) in fallow deer dental enamel. The new equation will allow direct comparison with other herbivore data. Variable δ¹⁸ O values within populations of fallow deer broadly reflect the ecological zones they are found in which may explain this pattern of results in other euryphagic species.

First Radiological Study of a Complete Dental Ontogeny Sequence of an Extinct Equid: Implications for Equidae Life History and Taphonomy

The sequence of cheek teeth mineralization, eruption, and replacement of an extinct horse species is here documented with radiological techniques for the first time thanks to the exceptional preservation of Hipparion sp. mandibles from Cerro de los Batallones (Madrid Basin, Spain). The sequence of dental ontogeny in mammals provides valuable insights about life history traits, such as the pace of growth, and about the mode of formation of fossiliferous assemblages. We have determined that the order of permanent cheek teeth mineralization and eruption of hipparionine horses is m1, m2, (p2, p3), p4, m3. Cheek teeth mineralization timing of hipparionine horses coincides with the one observed in modern equids. In turn, there are differences in the eruption timing of the p4 and m3 between horses belonging to the Anchitheriinae and Hipparionini compared to equids of the Equus genus that might be related to the shorter durability of the deciduous tooth dp4 in anchitheriine and hipparionine horses and, more broadly, to an increased durability of equid teeth through their evolutionary history. Based on the dental eruption sequence, hipparionine horses are slow-growing, long-living mammals. The Hipparion sp. assemblage from Batallones-10 conforms to an attritional model, as individuals more vulnerable to natural mortality predominate.

Ontogenetic and life history trait changes associated with convergent ecological specializations in extinct ungulate mammals

Investigating life history traits in mammals is crucial to understand their survival in changing environments. However, these parameters are hard to estimate in a macroevolutionary context. Here we show that the use of dental ontogenetic parameters can provide clues to better understand the adaptive nature of phenotypic traits in extinct species such as South American notoungulates. This recently extinct order of mammals evolved in a context of important geological, climatic, and environmental variations. Interestingly, notoungulates were mostly herbivorous and acquired high-crowned teeth very early in their evolutionary history. We focused on the variations in crown height, dental eruption pattern, and associated body mass of 69 notoungulate taxa, placed in their phylogenetic and geological contexts. We showed that notoungulates evolved higher crowns several times between 45 and 20 Ma, independently of the variation in body mass. Interestingly, the independent acquisitions of ever-growing teeth were systematically accompanied by eruption of molars faster than permanent premolars. These repeated associations of dental innovations have never been documented for other mammals and raise questions on their significance and causal relationships. We suggest that these correlated changes could originate from ontogenetic adjustments favored by structural constraints, and may indicate accelerated life histories. Complementarily, these more durable and efficient dentitions could be selected to cope with important ingestions of abrasive particles in the context of intensified volcanism and increasing aridity. This study demonstrates that assessing both life history and ecological traits allows a better knowledge of the specializations of extinct mammals that evolved under strong environmental constraints.