Abstract
Taphonomic deformation can make the interpretation of vertebrate fossil morphology difficult. The effects of taphonomic deformation are investigated in two ankylosaurid dinosaur taxa, Euoplocephalus tutus (to investigate effects on our understanding of intraspecific variation) and Minotaurasaurus ramachandrani (to investigate the validity of this genus). The ratio of orbit maximum rostrocaudal length to perpendicular height is used as a strain ellipse, which can be used to determine if ankylosaur skull fossils have been dorsoventrally compacted during fossilization and diagenesis. The software program Geomagic is used to retrodeform three-dimensional (3D) digital models of the ankylosaur skulls. The effects of sediment compaction are modeled using finite element analysis, and the resulting strain distributions are compared with the retrodeformed models as a test of the retrodeformation method. Taphonomic deformation can account for a large amount of intraspecific variation in Euoplocephalus, but finite element analysis and retrodeformation of Minotaurasaurus shows that many of its diagnostic features are unlikely to result from deformation.
Highlights
Variation among specimens referred to a single fossil taxon can originate from several biological sources, such as ontogeny, sexual dimorphism, and individual variation, but taphonomy can be a source of morphological variation in fossils
Plastic deformation of a fossil is more likely to occur during fossilization and diagenesis, during which time bone can act like a ductile material
This study examines two cases where understanding deformation can be used to better interpret ankylosaur cranial morphology: 1) intraspecific variation in Euoplocephalus tutus Lambe, 1910 [11], and 2) the taxonomic validity of Minotaurasaurus ramachandrani Miles and Miles, 2009 [12]
Summary
Variation among specimens referred to a single fossil taxon can originate from several biological sources, such as ontogeny, sexual dimorphism, and individual variation, but taphonomy can be a source of morphological variation in fossils. Bones can become deformed, and this deformation can lead to difficulties in understanding taxonomic variation, phylogenetic relationships, and functional morphology [1,2,3,4]. Understanding the effects of taphonomic deformation on bones is important for interpreting morphological variation. Fossils can become distorted from the effects of brittle or plastic deformation (or both). Whether or not a fossil undergoes brittle or plastic deformation is dependent on the temperature, confining pressure, and strain rate it experiences. Many fossils undergo brittle deformation prior to burial, cracking and fracturing during transport, and brittle deformation can occur during diagenesis as well, such as if a fossil is faulted. Plastic deformation of a fossil is more likely to occur during fossilization and diagenesis, during which time bone can act like a ductile material. The orientation of a specimen within the sediment will affect how the specimen deforms [6]
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