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The evolution of the cervid skull
The evolution of the cervid skull
Cervidae (deer) forms a large family of cud-chewing, even-toed mammals (Artiodactyla: Ruminantia). It is closely related to Moschidae (musk deer), Bovidae (cattle, goats, sheep and antelopes), Giraffidae (giraffes), and Antilocapridae (American pronghorns); Tragulidae (mouse-deer and chevrotains) com-prises somewhat more distant relatives. Cervids originated in the Early Miocene (~ 20 mya) in Europe. Cervids form a highly diversified group and occupy a wide range of ecological niches, from tropical forests to Arctic tundra. They may be found in Europe and Asia, and the Americas. In Africa, cervids are native only to a narrow range along the Mediterranean. No native cervids are known from Australia and Antarctica. This wide geographic and ecological distribution goes along with extensive adaptations. Extant cervids range in size from the small South American pudus, weighing about 6 kg to the large moose, weighing up to 600 kg. They also differ in their social behaviour, living either solitary or in male-dominated groups. However varied cervids may be, their best-known and diagnostic commonality is that males develop antlers which are shed and regularly regrown. The one exception is the water deer which lacks antlers but shows enlarged upper canines. A central theme of the present thesis is whether and how the development of antlers and the ecological diversification affected the structure of the skull. Specifically, we wanted to clarify how the size and shape of the facial facet of the lacrimal bone vary in extant cervids and two iconic extinct cervids from the last glacial. The lacrimal facial facet holds a central position between the frontal bone, where antlers arise, and the maxilla. We compared the size and shape of the lacrimal facial facet of ten extant cervid species using Moschus and Tragulus as outgroups. Neither size nor shape of this central bone of the facial skeleton could be related to the species' specific ecology or behaviour analysed. However, both measures were found to correlate with skull length. Size scales positively with skull length, as does the lacrimojugal length. In contrast, lacrimomaxillar length shows negative allometric scaling. We propose that during cervid evolution the lacrimal facial facet was exapted to dampen the transfer of stress originating from antlers to the maxilla. In the extinct “Irish Elk”, the giant deer Megaloceros, and the dwarf cervid from Crete, Candiacervus we observed quite small lacrimal facial facets relative to their skull length. Further, the shapes of the lacrimal facial facets are similar in the two fossil cervids but differ from those observed in extant cervids. We argue that these morphological differences in the lacrimal facial facet may suggest that Megaloceros and Candiacervus used their exceedingly large antlers as ornaments, and not also as weapons. In order to better interpret the quantitative data of the lacrimal facial facet in Pleistocene cervids, we re-assessed the cranial morphology of Candiacervus, taking advantage of eight skulls, hitherto un-described and housed at the SNSB-Bayerische Staatssammlung für Paläontologie und Geologie. Five of these skulls could be assigned to the large-antlered C. ropalophorus. An unexpected observation was that the dentition pattern of Candiacervus as suggested by the present sample differed from that typical for cervids. This may be tentatively interpreted as an adaptation to their specific insular environment. In all analyses of the cervid skull, the water deer, Hydropotes inermis, commands special attention because it is the only deer that has no antlers. An early challenge of the current research project was to collect and integrate data available for this peculiar but still poorly understood cervid. To this end, we conducted an extensive search of literature covering its biogeography, its physical appearance and morphology, ecology and behaviour, aspects of genetics and phylogenetics and the fossil record. The critical review and integration of these data not only constitute a solid basis for our comparison of the skull of H. inermis with those of antlered cervids, but it also provides a compact and compassing guide for anyone interested in cervid biology. As detailed in the final chapter and concluding discussion of this dissertation, a comprehensive assessment of the biology of H. inermis puts into question the recently proposed genetic mechanism underlying the antler-less status of this species. Arguably, unravelling the genetic basis of antler ontogenetic induction and phylogenetic origin, and its relation to the phylogenetic regression of upper canines should be a key to understanding the specifics and evolutionary diversification of the cervid skull.
Cervidae, morphology, exaptation, island, fossil deer, conservation biology
Schilling, Ann-Marie
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Schilling, Ann-Marie (2021): The evolution of the cervid skull. Dissertation, LMU München: Faculty of Geosciences
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Abstract

Cervidae (deer) forms a large family of cud-chewing, even-toed mammals (Artiodactyla: Ruminantia). It is closely related to Moschidae (musk deer), Bovidae (cattle, goats, sheep and antelopes), Giraffidae (giraffes), and Antilocapridae (American pronghorns); Tragulidae (mouse-deer and chevrotains) com-prises somewhat more distant relatives. Cervids originated in the Early Miocene (~ 20 mya) in Europe. Cervids form a highly diversified group and occupy a wide range of ecological niches, from tropical forests to Arctic tundra. They may be found in Europe and Asia, and the Americas. In Africa, cervids are native only to a narrow range along the Mediterranean. No native cervids are known from Australia and Antarctica. This wide geographic and ecological distribution goes along with extensive adaptations. Extant cervids range in size from the small South American pudus, weighing about 6 kg to the large moose, weighing up to 600 kg. They also differ in their social behaviour, living either solitary or in male-dominated groups. However varied cervids may be, their best-known and diagnostic commonality is that males develop antlers which are shed and regularly regrown. The one exception is the water deer which lacks antlers but shows enlarged upper canines. A central theme of the present thesis is whether and how the development of antlers and the ecological diversification affected the structure of the skull. Specifically, we wanted to clarify how the size and shape of the facial facet of the lacrimal bone vary in extant cervids and two iconic extinct cervids from the last glacial. The lacrimal facial facet holds a central position between the frontal bone, where antlers arise, and the maxilla. We compared the size and shape of the lacrimal facial facet of ten extant cervid species using Moschus and Tragulus as outgroups. Neither size nor shape of this central bone of the facial skeleton could be related to the species' specific ecology or behaviour analysed. However, both measures were found to correlate with skull length. Size scales positively with skull length, as does the lacrimojugal length. In contrast, lacrimomaxillar length shows negative allometric scaling. We propose that during cervid evolution the lacrimal facial facet was exapted to dampen the transfer of stress originating from antlers to the maxilla. In the extinct “Irish Elk”, the giant deer Megaloceros, and the dwarf cervid from Crete, Candiacervus we observed quite small lacrimal facial facets relative to their skull length. Further, the shapes of the lacrimal facial facets are similar in the two fossil cervids but differ from those observed in extant cervids. We argue that these morphological differences in the lacrimal facial facet may suggest that Megaloceros and Candiacervus used their exceedingly large antlers as ornaments, and not also as weapons. In order to better interpret the quantitative data of the lacrimal facial facet in Pleistocene cervids, we re-assessed the cranial morphology of Candiacervus, taking advantage of eight skulls, hitherto un-described and housed at the SNSB-Bayerische Staatssammlung für Paläontologie und Geologie. Five of these skulls could be assigned to the large-antlered C. ropalophorus. An unexpected observation was that the dentition pattern of Candiacervus as suggested by the present sample differed from that typical for cervids. This may be tentatively interpreted as an adaptation to their specific insular environment. In all analyses of the cervid skull, the water deer, Hydropotes inermis, commands special attention because it is the only deer that has no antlers. An early challenge of the current research project was to collect and integrate data available for this peculiar but still poorly understood cervid. To this end, we conducted an extensive search of literature covering its biogeography, its physical appearance and morphology, ecology and behaviour, aspects of genetics and phylogenetics and the fossil record. The critical review and integration of these data not only constitute a solid basis for our comparison of the skull of H. inermis with those of antlered cervids, but it also provides a compact and compassing guide for anyone interested in cervid biology. As detailed in the final chapter and concluding discussion of this dissertation, a comprehensive assessment of the biology of H. inermis puts into question the recently proposed genetic mechanism underlying the antler-less status of this species. Arguably, unravelling the genetic basis of antler ontogenetic induction and phylogenetic origin, and its relation to the phylogenetic regression of upper canines should be a key to understanding the specifics and evolutionary diversification of the cervid skull.