Helder Gomes Rodrigues

Enamel histology may provide valuable insights into dental growth and life history traits. However, this method presents significant challenges for extinct species, particularly as the periodicity of the increments observed cannot be tested experimentally. Here, we propose a standardized methodological framework to formalize what has been done in the literature and help researchers working with fossil taxa. We applied this framework to a fossil dataset composed of 19 molars belonging to four species of Toxodontia (Notoungulata, Mammalia) — an extinct sub‐order of South American notoungulates known for their specialized dental characteristics (crown height increase and change in eruption pattern) — with different crown height and age: the brachyodont species Pleurostylodon modicus from the Casamayor sub‐unit of the Sarmiento Formation (Eocene; Argentina), and the three hypsodont species Eurygenium pacegnum from Salla (Late Oligocene; Bolivia), Adinotherium ovinum and Nesodon imbricatus from the Santa Cruz Formation (Middle Miocene; Argentina). Our protocol starts with the identification of Retzius lines as increments reaching the tooth surface and forming perikyma grooves. Retzius lines were often observed in restricted areas of the cervical region and/or outer enamel, as previously reported in the literature and confirmed in Toxodontia. Increments between successive Retzius lines were identified as laminations, corresponding to daily features. Lastly, although rare, sub‐daily features in the form of cross‐striations or fine laminations were also observed. The daily secretion rate (DSR) ranged from 6 to 22 μm/day, while the enamel extension rate (EER) ranged from 13 to 376 μm/day and the enamel formation front angle (EFFa) varies from 3 to 30°. There were however great disparities, especially depending on species and on crown region. In particular, the EER decreased drastically (and the EFFa increased) between the occlusal and the cervical regions. This decrease was less marked from the middle to the cervical region in more recent toxodonts ( Adinotherium and Nesodon ), suggesting a cruising speed in tooth growth probably associated with hypsodonty and a longer crown formation time. To investigate interspecific differences, we thus focused on the middle region of the crown. We noted lower values of DSR and EER as well as higher values of EFFa in Pleurostylodon , the smallest, oldest and only brachyodont species of the dataset, compared with the other taxa. However, as our sample is limited taxonomically, we could not clearly distinguish between the relative roles of hypsodonty, body mass, and phylogeny on the histological parameters. This work paves the way for a robust study of a larger dataset of Notoungulata to explore their tooth growth and life history along their repeated evolution of dental specializations.

The growth of teeth and jaw bones is intimately linked in vertebrates, especially in mammals due to their specialized dentition and limited body growth. However, the relative patterns of growth and level of integration (i.e., co‐variation) of these structures are insufficiently known, which hinders our ability to understand how the jaw bones accommodate the diverse dental shapes and eruption patterns observed in mammals. Here, we studied the cranial ontogenetic series of 23 ungulate species among artiodactyls, perissodactyls, and hyracoids having different dental shapes and eruption patterns. We evaluated the variation of the teeth‐palate complex, as well as the co‐variation of teeth and palate during growth using 3D geometric morphometrics. We found a major ontogenetic component common to all the species studied, corresponding to an elongated dental row relative to the palate in juveniles and vice versa in adults. This pattern agrees with the prolonged growth of the palate as compared to teeth during development but is also reminiscent of an intraspecific allometric pattern previously observed in some dwarf ungulates. Moreover, most artiodactyls, especially ruminants, departed from other ungulates in having a higher co‐variation between the dental row and the palate. This stronger integration seen in ruminants might be associated with their inherited rapid growth and relatively fast eruption pattern. This is in contrast to ungulates with late eruption of last molars, whose teeth‐palate complex might be less constrained, but further investigation is needed to substantiate these hypotheses and better understand the factors influencing covariations within the upper jaw.

Dental anomalies are frequent in boars and pigs, and they generally affect the first premolar loci. The prevalence of these dental anomalies was investigated in a large number of populations around the world. These studies mainly focused on the influence of domestication, size, sexual dimorphism or food hardness on these anomalies. However, they rarely considered ontogenetic aspects, while these are crucial for understanding their aetiology during animal growth and how the dental row‐jaw complex is affected. Here, we studied the incidence of missing first upper and lower premolars in a French population of captive wild boars to discuss the functional and developmental reasons for missing teeth and to assess the impact of missing teeth on the growth of the dental row‐jaw complex. Using the CT‐scan data of the cranium and mandible of 24 wild boars investigated six times each during their growth, and presenting a balanced sex ratio, we recorded the number of missing teeth. We then quantified the shape of the upper and lower jaws using 3D geometric morphometrics. We found a similar prevalence of missing first premolar (37.5%) between the upper and the lower jaws, which is higher than the frequencies observed in most continental populations of wild boars. The increasing number of anomalies during ontogeny suggests a relaxed constraint on the dentition associated with a different feeding behaviour in captivity. The absence of first premolars does not appear to be associated with size variation or sexual dimorphism, nor does it affect the place of the dentition within the jaw, the latter being more influenced by the dimorphic shape of the canines and the timing of dental eruption.

Evolutionary changes in vertebrates are linked to genetic alterations that often affect tooth crown shape, which is a criterion of speciation events. The Notch pathway is highly conserved between species and controls morphogenetic processes in most developing organs, including teeth. Epithelial loss of the Notch-ligand Jagged1 in developing mouse molars affects the location, size and interconnections of their cusps that lead to minor tooth crown shape modifications convergent to those observed along Muridae evolution. RNA sequencing analysis revealed that these alterations are due to the modulation of more than 2000 genes and that Notch signaling is a hub for significant morphogenetic networks, such as Wnts and Fibroblast Growth Factors. The modeling of these tooth crown changes in mutant mice, via a three-dimensional metamorphosis approach, allowed prediction of how Jagged1-associated mutations in humans could affect the morphology of their teeth. These results shed new light on Notch/Jagged1-mediated signaling as one of the crucial components for dental variations in evolution.

Anatomical and biological specializations have been studied extensively in fossorial rodents, especially in subterranean species, such as mole‐rats or pocket‐gophers. Sciurids (i.e., squirrels) are mostly known for their diverse locomotory behaviors, and encompass many arboreal species. They also include less specialized fossorial species, such as ground squirrels that are mainly scratch diggers. The skull of ground squirrels remains poorly investigated in a fossorial context, while it may reflect incipient morphological specializations associated with fossorial life, especially due to the putative use of incisors for digging in some taxa. Here, we present the results of a comparative analysis of the skull of five fossorial sciurid species, and compare those to four arboreal sciurids, one arboreal/fossorial sciurid and one specialized fossorial aplodontiid. The quantification of both cranial and mandibular shapes, using three dimensional geometric morphometrics, reveals that fossorial species clearly depart from arboreal species. Fossorial species from the Marmotini tribe, and also Xerini to a lesser extent, show widened zygomatic arches and occipital plate on the cranium, and a wide mandible with reduced condyles. These shared characteristics, which are present in the aplodontiid species, likely represent fossorial specializations rather than relaxed selection on traits related to the ancestral arboreal condition of sciurids. Such cranial and mandibular configurations combined with proodont incisors might also be related to the frequent use of incisors for digging (added to forelimbs), especially in Marmotini evolving in soft to hard soil conditions. This study provides some clues to understand the evolutionary mechanisms shaping the skull of fossorial rodents, in relation to the time spent underground and to the nature of the soil.

Comment se faire les dents en paléontologie ? was published in Paléontologie d'aujourd'hui on page 192.