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S. J. Rahmat and I. A. Koretsky

Abstract

Comparison of mandibular morphology of some aquatic (seals, walruses, and sea otters) and terrestrial (hyenas and pandas) carnivorans demonstrates a rather general pattern correlating size of condyloid angle, size of gape, and diet. Structural differences of carnivoran jaws reveal morphological and ecological adaptations that are directly correlated with availability of prey, diving depth, feeding competition and specialized feeding methods. Specifically, the inclination of the condyloid process relative to the axis of the alveolar row (= condyloid angle) can be used to determine dietary preferences, including size of prey. Generally, carnivorans with a large condyloid angle feed on larger prey, while a low condyloid angle suggests feeding on small prey or can be an advantageous feeding mechanism. Mirounga angustirostris (Northern elephant seal) displays sex-specific characters in cranial and postcranial elements. Likewise, significant sexually dimorphic differences in the size of condyloid angle imply that deeper-diving male Northern elephant seals have a feeding niche dissimilar to that of females. Morphological assessment of male M. angustirostris suggests they are bottom-feeding seals that utilize a suction-feeding mechanism to capture small prey and crush shells with their teeth, which become weaker as they age.

Open access

I. A. Koretsky, L. G. Barnes and S. J. Rahmat

Abstract

The origin of pinnipeds has been a contentious issue, with opposite sides debating monophyly or diphyly. This review uses evidence from the fossil record, combined with comparative morphology, molecular and cytogenetic investigations to evaluate the evolutionary history and phylogenetic relationships of living and fossil otarioid and phocoid pinnipeds. Molecular investigations support a monophyletic origin of pinnipeds, but disregard vital morphological data. Likewise, morphological studies support diphyly, but overlook molecular analyses. This review will demonstrate that a monophyletic origin of pinnipeds should not be completely accepted, as is the current ideology, and a diphyletic origin remains viable due to morphological and paleobiological analyses. Critical examination of certain characters, used by supporters of pinniped monophyly, reveals different polarities, variability, or simply convergence. The paleontological record and our morphological analysis of important characters supports a diphyletic origin of pinnipeds, with otarioids likely arising in the North Pacific from large, bear-like animals and phocids arising in the North Atlantic from smaller, otter-like ancestors. Although members of both groups are known by Late Oligocene time, each developed and invaded the aquatic environment separately from their much earlier, common arctoid ancestor. Therefore, we treat the superfamily Otarioidea as being monophyletic, including the families Enaliarctidae, Otariidae (fur seals/sea lions), Desmatophocidae, and Odobenidae (walruses and extinct relatives), and the superfamily Phocoidea as monophyletic, including only the family Phocidae, with four subfamilies (Devinophocinae, Phocinae, Monachinae, and Cystophorinae).

Open access

S. J. Rahmat and I. Koretsky

Abstract

The Devinophoca emryi material from the early Badenian, early Middle Miocene (16.26–14.89 Ma) presents mixed cranial and especially postcranial characters with the three extant phocid subfamilies (Cystophorinae, Monachinae and Phocinae), as well as unique postcranial characters not seen in any taxa. These distinguishing characters (i. e. well-outlined, large oval facet on greater tubercle of humerus; broader width between the head and lesser tubercle of humerus; femoral proximal epiphysis larger than distal; thin innominate ilium that is excavated on ventral surface) demonstrate that this material belongs to a recently described species (D. emryi). During ecomorphotype analyses, fossil humerus and femur bones were directly associated with their corresponding mandible to reveal associations based on Recent morphological analogues. Strong correlation between ecomorphotypes and postcranial morphology supports placement of this material to D. emryi and not its sister taxon, D. claytoni. The previously described skull, mandible and teeth and postcranial bones described herein were discovered at the same locality during excavations at the base of the Malé Karpaty Mountains (Slovakia), at the junction of the Morava and Danube rivers. The geological age of D. emryi and the presence of mixed characters strongly suggest that this species was an early relative to the ancestor of seals, possibly being a terminal branch of the phocid tree. This material allows for emended diagnoses of the species, updated assessments of geographical distribution and provides further material for clarification of controversial phylogenetic relationships in Phocidae.

Open access

I.A. Koretsky, S.J. Rahmat and N. Peters

Abstract

Remarks on Correlations and Implications of the Mandibular Structure and Diet in Some Seals (Mammalia, Phocidae). Koretsky, I. A., Rahmat, S. J., Peters, N. — The diverse representatives of Recent seals within the three extant subfamilies (Cystophorinae, Phocinae, Monachinae) of the family Phocidae exhibit dietary variations among species, feeding on invertebrates and a diversity of prey depending on the season and availability. To explain this variability, an introductory morphological examination of the mandibular structure of Recent seals from each subfamily was performed, focusing on: heights of the mandibular ramus and condyloid process; measurements of the masseteric fossa; and attachments of muscles of mastication. After measuring the condyloid angles (the inclination of the condyloid process in relation to the axis of the alveolar row) among the examined species, a correlation between the size ofthe angle, function, and diet was recognized. Seals with a tall mandibular ramus and greater condyloid angle (Cystophorinae) feed on larger-sized prey, while seals with a shorter ramus and lesser condyloid angle feed on small (Monachinae) to medium-sized (Phocinae) prey, regardless of the overall size of the seal. This study focused on the mandibular morphology of some living and fossil representatives of Phocidae, providing an association between functional and ecological interpretations of modern seals in general and extrapolating this knowledge for fossil dietary preference.

Open access

S. J. Rahmat, I. A. Koretsky, J. E. Osborne and A. A. Alford

Abstract

The Family Phocidae includes four subfamilies (Phocinae, Monachinae, Cystophorinae, and Devinophocinae) consisting of mediumto large-sized mammals that possess distinctive adaptations to semi-aquatic life. In the Miocene of the Chesapeake Group, only two subfamilies of the Family Phocidae were identified: Phocinae and Monachinae. Leptophoca, a representative of the subfamily Phocinae, appears on the eastern shore of the North Atlantic around 16 million years ago. Recently, two new monachine species, the larger Terranectes magnus (n. gen., n. sp.) and the medium-sized T. parvus (n. sp.), were recorded in the Upper Miocene of the Chesapeake Group in the Eastover Formation (7.0–6.0 Ma) and St. Marys Formation (10.0-8.0 Ma). These two distinct subfamilies of seals indicate a well-marked divergence between phocines and monachines, much earlier than 18 million years ago, as previously suggested. The Eastover Formation was deposited in a shallow embayment that covered southern Maryland, the coastal plain of Virginia, and the northeastern corner of North Carolina. The geologically older St. Marys Formation represents a tide-influenced coastal environment, with low-salinity estuaries. There was a sharp temperature decrease in the Late Miocene, indicated by a shift to a cooler-water fish fauna during St. Marys time. The Eastover Formation reflects warmer waters with relatively strong currents, significant shoals, barriers, and varied depths. Fossil evidence of earlier seals suggests that phocids originated in the North Atlantic and otarioids in the North Pacific. True seals diverged from ancient Carnivora in the early Oligocene (or earlier) in the Paratethyan / Mediterranean Basins, spread widely during the Middle Miocene and crossed westward across the Atlantic Ocean, before dispersing in the eastern United States by the Early Pliocene.