Daryl P. Domning

Bone histology of the ribs of the archaeocetes (Mammalia: Cetacea)

ABSTRACT Histological study of the ribs in two archaeocetes, Basilosaurus cetoides and Zygorhiza kochii, reveals conspicuous pachyostosis, with hyperostosis of periosteal cortices and absence of a free medullary cavity. In addition, abundant remains of calcified cartilage matrix, associated with extensive globular ossification, exist within the medulla. Among Cetacea, these features seem to be unique to the archaeocetes. Conversely, the structural peculiarities of bone in these animals are identical to those of the Sirenia and would arise from similar causes: inhibition of the chondroclastic and endosteal osteoclastic resorptions, and prolongation of subperiosteal osteogenesis during ontogeny, exemplifying morphological adaptation to aquatic life via heterochrony. The possible hormonal mechanisms of these features, as also their supposed adaptive consequences for the buoyancy of the archaeocetes, are discussed in relation to data already available for the Sirenia.

The role of phenacodontids in the origin of the modern orders of ungulate mammals

ABSTRACT We investigate the role of the ancestral ungulates Phenacodontidae in the evolution of Perissodactyla, Hyracoidea, Proboscidea, Sirenia, and Desmostylia on the basis of parsimony analysis of dental, cranial, and postcranial data. The monophyletic order Phenacodonta (Phenacodontidae and Meniscotheriidae) is the sistergroup to the mirorder Pantomesaxonia (consisting of the five mammalian orders mentioned), and these two clades are united as superorder Paenungulata. The relationships of taxa within Pantomesaxonia remain unresolved. Important characters of the ancestor of Pantomesaxonia include partially molarized premolars and a cursorial limb skeleton.

Fossil sirenia of the west Atlantic and Caribbean region. V. The most primitive known sirenian, Prorastomus sirenoides Owen, 1855

ABSTRACT Prorastomus sirenoides is both the oldest and the most primitive sirenian known; it is the only unquestioned sirenian recorded from beds of probable early Eocene age. The unique holotype skull from Jamaica is here redescribed, new material from middle Eocene beds in Jamaica is referred to this species, and two atlases from the late Eocene of Florida are referred to the family Prorastomidae. Although Prorastomus appears to have some autapomorphies that exclude it from the direct ancestry of other sirenians, it is the best available approximation to a structural ancestor of the Sirenia. Therefore, Prorastomus is more appropriate than any Recent species as a representative of the order in analyzing the relationships of sirenians to other mammals. Sirenians are cladistically related to some other primitive ungulates as follows: (Arctocyon (Phenacodus (Moeritherium, Sirenia))). The 3.1.5.3 dental formula of Prorastomus and other Eocene sirenians may be a synapomorphy of the order and a reversal of the...

Evolution of Sirenian Pachyosteosclerosis, a Model-case for the Study of Bone Structure in Aquatic Tetrapods

Osteosclerosis, or inner bone compaction, and pachyostosis, or outer hyperplasy of bone cortices (swollen bones), are typical features of tetrapods secondarily adapted to life in water. These peculiarities are spectacularly exemplified by the ribs of extant and extinct Sirenia. Sea cows are thus the best model for studying this kind of bone structural specializations. In order to document how these features differentiated during sirenian evolution, the ribs of 15 species, from the most basal form (Pezosiren portelli) up to extant taxa, were studied, and compared to those of other mammalian species from both morphometric and histological points of view. Pachyostosis was the first of these two specializations to occur, by the middle of the Eocene, and is a basal feature of the Sirenia. However, it subsequently regressed in some taxa that do not exhibit hyperplasic rib cortices. Osteosclerosis was only incipient in P. portelli. Its full development occurred later, by the end of the Eocene. These two structural specializations of bone are variably pronounced in extinct and extant sirenians, and relatively independent from each other, although frequently associated. They are possibly due to similar heterochronic mechanisms bearing on the timing of osteoblast activity. These results are discussed with respect to the functional constraints of locomotion in water.

Iterative Evolution of Sympatric Seacow (Dugongidae, Sirenia) Assemblages during the Past ∼26 Million Years

Extant sirenians show allopatric distributions throughout most of their range. However, their fossil record shows evidence of multispecies communities throughout most of the past ∼26 million years, in different oceanic basins. Morphological differences among co-occurring sirenian taxa suggest that resource partitioning played a role in structuring these communities. We examined body size and ecomorphological differences (e.g., rostral deflection and tusk morphology) among sirenian assemblages from the late Oligocene of Florida, early Miocene of India and early Pliocene of Mexico; each with three species of the family Dugongidae. Although overlapping in several ecomorphological traits, each assemblage showed at least one dominant trait in which coexisting species differed. Fossil sirenian occurrences occasionally are monotypic, but the assemblages analyzed herein show iterative evolution of multispecies communities, a phenomenon unparalleled in extant sirenian ecology. As primary consumers of seagrasses, these communities likely had a strong impact on past seagrass ecology and diversity, although the sparse fossil record of seagrasses limits direct comparisons. Nonetheless, our results provide robust support for previous suggestions that some sirenians in these extinct assemblages served as keystone species, controlling the dominance of climax seagrass species, permitting more taxonomically diverse seagrass beds (and sirenian communities) than many of those observed today.

Structure, Function, and Adaptation of the Manatee Ear

The West Indian manatee, Trichechus manatus, is common throughout the Caribbean yet little is known about its sensory abilities. Hearing in particular is of interest since large numbers of one subspecies, the Florida manatee (T. manatus latirostris), die annually from collisions with boats in shallow coastal waters and canals. There is no published audiogram for Trichechus manatus and the auditory system has not been fully described. Earlier studies of manatee hearing were based on isolated, dehydrated tympano-periotic bones (Robineau, 1969; Fleischer, 1978). This paper describes all major hard and soft tissues of the peripheral auditory system of T. manatus, presents new information on specialized cranial features that may be important for sound conduction, and provides morphometrybased estimates of the frequency range and sensitivity of West Indian manatee ears.

Use of tusks in feeding by dugongid sirenians: observations and tests of hypotheses.

Most living and fossil sea cows of the subfamily Dugonginae (Dugongidae, Sirenia, Mammalia) are characterized by large upper incisor tusks, which are thought to play an important role (at least primitively) in feeding on seagrass rhizomes. Testing this hypothesis is difficult, because the only extant tusked sirenian (Dugong dugon) is morphologically and perhaps behaviorally aberrant. The tests attempted here involve examination of stomach contents of wild Recent dugongs, experiments using plastic replicas of diverse tusks to harvest seagrasses, gross anatomical observations on tusks and skulls, measurements of tusk tip geometry, and observations of microwear on tusks. We conclude that (a) male D. dugon (with erupted tusks) do not consume more rhizomes than females (without erupted tusks); (b) the tusks do not play a significant role in feeding in the modern dugong; (c) larger, more bladelike tusks are more effective at harvesting rhizomes, but the effect of shape was not experimentally separated from the effect of exposed tusk length; (d) some fossil dugongines show apparent cranial adaptations for downward and backward cutting motions of their large, bladelike tusks; (e) geometry of wear surfaces is consistent with use of at least the more bladelike tusks as cutting instruments; (f) preliminary observations of microwear in D. dugon do not indicate more than occasional use of the tusks in purposeful harvesting of rhizomes, and then only opportunistically by large adult males. The hypothesis of such tusk use by extinct dugongines (in contrast to the living species) is so far corroborated, but available data and tests do not suffice to establish this conclusively. Anat Rec, 290:523–538, 2007.

Sea cows and sea grasses

The review of current developments in paleobotany by Knoll and Rothwell (1981) prompts some supplementary observations on the state of marine paleobotany and, more generally, the problem of investigating groups with a poor fossil record. The marine angiosperms and the larger noncalcareous algae are such groups, being represented by very few fossils widely scattered in time and space (den Hartog 1970; Parker and Dawson 1965). Tropical freshwater macrophytes are likewise poorly known as fossils (Sculthorpe 1967).

Distribution and status of manatees Trichechus spp. Near the mouth of the amazon river, Brazil

Abstract Interviews with hunters and collection of skulls indicate that Trichechus inunguis occurs throughout the region of the Amazon River estuaries from Amapa to the mainland of Para, including Ilha de Marajo and islands on its Atlantic coast. T. manatus has a disjunct distribution in Brazil, apparently occurring both on the coast of Amapa north of Cabo Norte and in the Rio Mearim (Maranhao) as well as further to the southwest; it seems to have been exterminated from the Atlantic coast of Para and is absent from the Marajo region. Continued subsistence hunting can best be controlled by destruction of illegal camboas (fence-like traps) which catch manatees at high tide. Areas deserving study are the coast and inland lakes of eastern Amapa, the only place in the world where two sirenian species might still be found in sympatry or immediate proximity, and the lower Rio Mearim in Maranhao, which may still contain a sizable population of T. manatus. Ecological studies in these areas should have particular relevance to hypotheses of sirenian evolutionary interactions, and manatee reserves should be established in both areas.

Fossil Sirenia of the West Atlantic and Caribbean region. I. Metaxytherium floridanum Hay, 1922

ABSTRACT The vast majority of the abundant fossil sirenian remains from the Bone Valley phosphate mining district (chiefly in Polk and Hillsborough counties, Florida) represent a single species of dugongid, Metaxytherium floridanum Hay, 1922. The osteology of this species is described here in detail. Most or all of these specimens apparently come from Units 4–5 of the lower Bone Valley Formation and are late middle Miocene (early Clarendonian) in age. M. ossivallense (Simpson, 1932) is a junior subjective synonym of M. floridanum. M. calvertense Kellogg, 1966, and “Hesperosiren” Simpson, 1932, are not represented in the Bone Valley Formation. Specimens are also reported here from elsewhere in Florida, mainly Alachua County. Some or all of these are later than the typical Bone Valley form, being of late Clarendonian or early Hemphillian age, and some of them, here termed Metaxytherium, cf. M. floridanum, seem to represent a slightly more advanced stage of evolution. M. floridanum had a strongly deflected r...