Browsing by Subject "Morphometrics"
Item Geometric morphometrics of Antillean Crocodiles(2012-08) Weaver, Jeremy; Densmore, Llewellyn D.; Bradley, Robert D.; Strauss, Richard E.Geometric morphometrics commonly have been utilized to explore patterns of variation across a wide range of taxa. We present a geometric morphometric analysis of skull shape morphology for New World crocodilians of the genus Crocodylus, placing emphasis on studying variation within the Greater Antillean region of the Neotropics. It has been suggested that the major factor contributing to the modern diversity of Cuban (C. rhombifer) and American crocodiles (C. acutus) in the Greater Antilles is the result of ancient hybridization. Genetic studies found that mitochondrial DNA haplotypes for C. acutus in the Greater Antilles are actually more closely related to C. rhombifer than other American crocodiles throughout the Neotropics. To infer whether genetic relationships are correlated with morphological relationships, we use geometric morphometrics to assess shape variation and compare skull morphology to a reconstruction of a cytochrome-b gene phylogeny. Analysis of skull shape variation using geometric morphometrics of landmark data reveals three broad groups of New World Crocodylus within the given morphospace. Two of these groups correspond to present day Crocodylus whereas the other corresponds to fossil specimens of C. rhombifer. Within these groups, nearly all sub-groups correspond to our current taxonomic understanding of New World Crocodylus; except for the placement of Greater Antillean C. acutus, which clusters much closer to C. rhombifer. This further supports recent studies of Greater Antillean C. acutus dynamics and their genetic phylogenies, indicating a unique evolutionary history.Item Landmark morphometric assessment of the genus Baiomys (Mammalia: Rodentia): Geographic variation, secondary sexual dimorphism, and character displacement in sister species(2006-05) Abuzeineh, Alisa A.; Owen, Robert D.; McIntyre, Nancy E.; Bradley, Robert D.The purpose of this research was to evaluate geographic variation and secondary sexual dimorphism within the two recognized species of Baiomys (Rodentia: Muridae), and to determine whether (and to what extent) character displacement occurs within either or both of these species where they are found in sympatry. The genus Baiomys is composed of two extant species, B. taylori and B. musculus, which are quite similar in appearance. The two species are distinguishable by a limited number of cranial features (occipitonasal length, zygomatic breadth, and curvature of rostrum), hyoid, and baculum. Landmark point coordinates were collected from dorsal and ventral views of the crania and mandibles of 513 specimens representing the geographic distribution of both species. Geometric morphometric techniques were used to examine sexual dimorphism and geographic variation within each species, and to determine whether character displacement was present within the area of sympatry. A discriminant function analysis of Baiomys taylori showed a significant difference between the two sexes (p=0.03), whereas B. musculus did not (p=0.15). Additionally, a discriminant analysis showed significant differences (p=0.05) among four pre-designated groups (B. taylori allopatric and sympatric, and B. musculus allopatric and sympatric). The only significant pair-wise difference occurred between B. taylori sympatric and allopatric populations, whereas the sympatric B. taylori and B. musculus populations were quite similar to each other. A correlation coefficient (r) of 0.45 between morphometric variation and latitude was found for both species and sexes combined, and the slopes of the two species were not significantly different from each other (p=0.46). Furthermore, it was discovered that interpoint distances (¡Ö size) in the cranium and mandible increased in lower latitudes in both species, so that the largest B. taylori and smallest B. musculus were found in the zone of sympatry. This high correlation between morphological score and latitude across both species, together with the results of the discriminant analysis, indicate that character displacement does not occur in cranial and mandibular characters in Baiomys species. These findings are contrary to early speculations that character displacement was present in the genus.Item Sutural form and shell morphology of Placenticeras and systematic descriptions of Late Cretaceous ammonites from the Big Bend region, Texas(2006-08) Waggoner, Karen J.; Lehman, Thomas; Barrick, James E.; Chatterjee, Sankar; Strauss, Richard E.; Mulligan, KevinTwo related investigations document the sutural form, shell morphology, and taxonomy of several Late Cretaceous ammonites. A morphometric study utilizes geographical information systems (GIS) and multivariate statistical tools to examine the taxonomy, phylogeny, and biostratigraphic utility of ten Placenticeras species common in North American Cretaceous strata. A second biostratigraphic study provides systematic descriptions of ammonites collected from Upper Cretaceous (Campanian) strata in the Big Bend and Sierra Vieja areas of West Texas, using the GIS and statistical methods employed in the first study to assist with identification of the Placenticeras specimens. A high degree of intraspecific variation and strong interspecific similarity among the ten Placenticeras species results in gradational shell morphology and sutural form to such an extent that confident discrimination is only possible for the extreme end members of the gradational series of species. Two distinct sutural lineages are recognized when GIS and morphometric analyses are based on sutural similarity, disregarding previous species assignment. The two separate sutural lineages persisted through most of Late Cretaceous time while shell morphology varied widely. This suggests that sutural form may be a more conservative character, whereas most aspects of shell morphology are more plastic. The Pen and Aguja formations in the Big Bend area and the Ojinaga and San Carlos formations in the Sierra Vieja area preserve ammonites representative of the Scaphites hippocrepis III zone, the Submortoniceras tequesquitense zone, the Menabites (Delawarella) delawarensis zone, and the Baculites maclearni zone. These Campanian biostratigraphic zones allow for correlation of Big Bend and Sierra Vieja strata with Gulf Coast and Western Interior strata. The two sutural form lineages documented in the morphometric study are also recognized in four species of Placenticeras represented in the Big Bend and Sierra Vieja collections. The importance of sutural form for examining the phylogeny of North American Placenticeras species suggests that future application of the methods utilized in this study to specimens representing various ontogenetic stages collected from well constrained stratigraphic successions may result in a complete revision of the genus Placenticeras.Item Sutural form and shell morphology of Placenticeras and systematic descriptions of Late Cretaceous ammonites from the Big Bend region, Texas(Texas Tech University, 2006-08) Waggoner, Karen J.; Lehman, Thomas; Barrick, James E.; Mulligan, Kevin; Strauss, Richard E.; Chatterjee, SankarTwo related investigations document the sutural form, shell morphology, and taxonomy of several Late Cretaceous ammonites. A morphometric study utilizes geographical information systems (GIS) and multivariate statistical tools to examine the taxonomy, phylogeny, and biostratigraphic utility of ten Placenticeras species common in North American Cretaceous strata. A second biostratigraphic study provides systematic descriptions of ammonites collected from Upper Cretaceous (Campanian) strata in the Big Bend and Sierra Vieja areas of West Texas, using the GIS and statistical methods employed in the first study to assist with identification of the Placenticeras specimens. A high degree of intraspecific variation and strong interspecific similarity among the ten Placenticeras species results in gradational shell morphology and sutural form to such an extent that confident discrimination is only possible for the extreme end members of the gradational series of species. Two distinct sutural lineages are recognized when GIS and morphometric analyses are based on sutural similarity, disregarding previous species assignment. The two separate sutural lineages persisted through most of Late Cretaceous time while shell morphology varied widely. This suggests that sutural form may be a more conservative character, whereas most aspects of shell morphology are more plastic. The Pen and Aguja formations in the Big Bend area and the Ojinaga and San Carlos formations in the Sierra Vieja area preserve ammonites representative of the Scaphites hippocrepis III zone, the Submortoniceras tequesquitense zone, the Menabites (Delawarella) delawarensis zone, and the Baculites maclearni zone. These Campanian biostratigraphic zones allow for correlation of Big Bend and Sierra Vieja strata with Gulf Coast and Western Interior strata. The two sutural form lineages documented in the morphometric study are also recognized in four species of Placenticeras represented in the Big Bend and Sierra Vieja collections. The importance of sutural form for examining the phylogeny of North American Placenticeras species suggests that future application of the methods utilized in this study to specimens representing various ontogenetic stages collected from well constrained stratigraphic successions may result in a complete revision of the genus Placenticeras.