Palaeobiology of tragulids (Mammalia: Artiodactyla: Ruminantia)

Guzmán Sandoval, Jonathan (2018): Palaeobiology of tragulids (Mammalia: Artiodactyla: Ruminantia). Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Tragulidae is a family of the order Artiodactyla and suborder Ruminantia. Its representatives are closely related to deer, antelopes and their relatives. Fossil evidence from Asia suggests an origin of the Tragulidae in the Eocene at least 34 million years ago, with a climax during the Miocene and subsequent decline until present. They were well represented with over 30 species grouped in the genera Archaeotragulus, Afrotragulus, Iberomeryx, Dorcabune, Dorcatherium, Siamotragulus, and Yunnanotherium, which were completely replaced by the infraorder Pecora. Fossil tragulids had a broad geographical distribution in Asia, Africa, and Europe, including different morphotypes (small to large species) and diverse diet preferences (e.g. intermediate feeders, browsers and grazers). Nowadays tragulids are represented by only ten species classified in the genera Tragulus, Moschiola and Hyemoschus. They are exclusively distributed in the Indo-Malayan and Afro-tropical regions and feeding predominantly on fruits. Compared with their fossil relatives, their restricted recent distribution, low species richness and similar phenotype have stimulated the idea that recent tragulids represent “living fossils”. However, comparative studies between fossil and living relatives are very rare, and even the idea that living tragulids have become “frozen” in their evolution has been recently questioned. In this context, the aim of this work is an analysis of the body mass and the morphological diversity of the dentition and skulls of tragulids through time including fossil and living species. Among the studied taxa, the detailed description of a so far unstudied tragulid material from the Miocene of Pakistan has completed previous knowledge on ancient diversity. The methodology used here comprised comparative morphometrics of teeth and skulls as well as digital 3D reconstruction of a fossil skull. The results evidence a broad range of body sizes (from 1.0 kg to more than 100.0 kg) amongst fossil species, contrasting with the limited size range (1.0 kg to 15.0 kg) of the living species. From the late Eocene to present, the analysis of median body mass per 2.0 mya showed a peak between 16.0 mya to 4.0 mya, including species with less than 17.9 kg (that includes the range of living species) and between 18.0 kg to 34.9 kg (only fossil species) as the most diverse class. If we consider the food preferences of fossil tragulids (≤ 17.9 kg and medium-sized species: 18.0 kg-34.9 kg), all categories from grazers, browsers and intermediate feeders have been reported, but not frugivores as in the living species. The study of the skulls of extant tragulids helped to understand their character disparity. With this, a hitherto undocumented difference in the neurocranium geometry among Asian tragulids was quantified: Tragulus javanicus and T. kanchil are relatively tall and their neurocrania are globose, while the heigth of the skull is less pronounced in T. napu and Moschiola spp and somewhat flattened, more comparable to Hyemoschus aquaticus of Africa. Here, it is hypothesized that a flatter skull might be related to dietary habits and mastication movements and/or to diving and under-water locomotion to escape from predator as previously reported. In addition, I have assembled a chart providing revised taxonomic assessments of the specimens of Moschiola and Tragulus included in my analysis. The descriptions of the fossil skull of Dorcatherium crassum improve the understanding on its external morphology as well as its affinities with living species. This fossil has a strong neurocranium with some hyper-developed elements, such as the sagittal and nuchal crests as well as highlight the canine tooth and its alveola. The general morphology is similar to living species, but its hyper-developed morphology is not comparable to living ones, and was probably adapted morpho-functionally to the acquisition and processing of hard food items. Compared with Tragulus, Moschiola and Hyemoschus, the skull of Dt. crassum is bigger, but it is similar in some bone proportions calculated here. In general Dt. crassum is more similar to H. aquaticus than to other living species, supporting their close affinity as reported in previous studies based on other characters (limbs, teeth, partial skulls). The flattened skull of both species support this close affinity. A formerly unreported great morphological diversity in the dentition of fossil tragulids was documented by an analysis of material from the Miocene of Pakistan. Thus, of seven species previously known from the Siwaliks, based on the height of the tooth crown and related morphological features, only four are here recognized: Dorcatherium nagrii, Dt. minus, Dt. majus and Dorcabune anthracotheroides. In addition, we extend the diversity with the new species Dorcatherium dehmi and unexpectedly we extend the distribution into the Siwaliks for Dt. naui and Dt. guntianum previously recorded exclusively from Europe. Thus, the detailed analysis of morphology and the morphometric variables enabled distinguishing tragulid species that were previously masked by overlapping size. Accordingly, the variation described above coincides with data from the limited literature and it is in line with other groups previously misinterpreted as ‘living ancestors’ or ‘living fossils’, which were shown to be part of a greater morphological diversity than previously thought. However, considering that ancient diversity of tragulids was greater than the one of current representatives, and the living species belong to a group that is mostly extinct and such, by definition, provide deficient samples, it is difficult to assess with these data, which are plesiomorphic or derived characters. However, my results suggest a similar palaeobiology in fossil and living tragulids. Finally, the outcomes presented in this Ph.D. thesis clearly enhance the understanding of morphological diversity and palaeobiology of these mammals, but at the same time, reinforce the importance of studies on tragulids in order to improve the understanding on their origin and evolution.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 20. Dezember 2018
• 1. Bericht­erstatter:in: Rößner, Gertrud

Fußnote

DOI Forschungsdaten: 10.5282/ubm/data.179