Jorge Velez-Juarbe

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.

Repeated mass strandings of Miocene marine mammals from Atacama Region of Chile point to sudden death at sea

Marine mammal mass strandings have occurred for millions of years, but their origins defy singular explanations. Beyond human causes, mass strandings have been attributed to herding behaviour, large-scale oceanographic fronts and harmful algal blooms (HABs). Because algal toxins cause organ failure in marine mammals, HABs are the most common mass stranding agent with broad geographical and widespread taxonomic impact. Toxin-mediated mortalities in marine food webs have the potential to occur over geological timescales, but direct evidence for their antiquity has been lacking. Here, we describe an unusually dense accumulation of fossil marine vertebrates from Cerro Ballena, a Late Miocene locality in Atacama Region of Chile, preserving over 40 skeletons of rorqual whales, sperm whales, seals, aquatic sloths, walrus-whales and predatory bony fish. Marine mammal skeletons are distributed in four discrete horizons at the site, representing a recurring accumulation mechanism. Taphonomic analysis points to strong spatial focusing with a rapid death mechanism at sea, before being buried on a barrier-protected supratidal flat. In modern settings, HABs are the only known natural cause for such repeated, multispecies accumulations. This proposed agent suggests that upwelling zones elsewhere in the world should preserve fossil marine vertebrate accumulations in similar modes and densities.

Isthminia panamensis, a new fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the evolution of 'river dolphins' in the Americas.

In contrast to dominant mode of ecological transition in the evolution of marine mammals, different lineages of toothed whales (Odontoceti) have repeatedly invaded freshwater ecosystems during the Cenozoic era. The so-called ‘river dolphins’ are now recognized as independent lineages that converged on similar morphological specializations (e.g., longirostry). In South America, the two endemic ‘river dolphin’ lineages form a clade (Inioidea), with closely related fossil inioids from marine rock units in the South Pacific and North Atlantic oceans. Here we describe a new genus and species of fossil inioid, Isthminia panamensis, gen. et sp. nov. from the late Miocene of Panama. The type and only known specimen consists of a partial skull, mandibles, isolated teeth, a right scapula, and carpal elements recovered from the Piña Facies of the Chagres Formation, along the Caribbean coast of Panama. Sedimentological and associated fauna from the Piña Facies point to fully marine conditions with high planktonic productivity about 6.1–5.8 million years ago (Messinian), pre-dating the final closure of the Isthmus of Panama. Along with ecomorphological data, we propose that Isthminia was primarily a marine inhabitant, similar to modern oceanic delphinoids. Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid. This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea. Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.

Fossil Sirenia of the West Atlantic and Caribbean region. Ix. Metaxytherium albifontanum, sp. nov.

ABSTRACT We describe a new species of the halitheriine dugongid genus Metaxytherium from the late Oligocene of Florida and South Carolina. The new species is represented by cranial and postcranial material, including parts of the axial and appendicular skeleton. Metaxytherium albifontanum, sp. nov., differs from other species of Metaxytherium by the following unique combination of plesiomorphic and derived characters: posterior end of nasal process of premaxilla broad and flat relative to what is observed in most other members of the genus (somewhat resembling M. subapenninum); ventral extremity of jugal under posterior edge of orbit (character 85[1]) (shared with M. krahuletzi); exoccipitals separated in dorsal midline (character 66[1]) (shared with all other species in the genus, except some M. krahuletzi); and innominate with acetabulum (nearly lost or lost in M. crataegense, M. floridanum, M. serresii). This new species was sympatric with two dugongines, Crenatosiren olseni and Dioplotherium manigaulti. The small tusks and cranial morphology of M. albifontanum, sp. nov., indicate that it was likely a consumer of small seagrasses. Our phylogenetic analysis is consistent with previous ones in placing Hydrodamalinae within a paraphyletic Metaxytherium spp. and placing the Metaxytherium spp. + Hydrodamalinae clade as the sister group to Dugonginae. Metaxytherium albifontanum, sp. nov., is the oldest known member of its genus; this might indicate that the group originated in the West Atlantic and Caribbean region and later dispersed to the Old World Tethys region.

Interordinal gene capture, the phylogenetic position of Steller's sea cow based on molecular and morphological data, and the macroevolutionary history of Sirenia.

The recently extinct (ca. 1768) Stellers sea cow (Hydrodamalis gigas) was a large, edentulous North Pacific sirenian. The phylogenetic affinities of this taxon to other members of this clade, living and extinct, are uncertain based on previous morphological and molecular studies. We employed hybridization capture methods and second generation sequencing technology to obtain >30kb of exon sequences from 26 nuclear genes for both H. gigas and Dugong dugon. We also obtained complete coding sequences for the tooth-related enamelin (ENAM) gene. Hybridization probes designed using dugong and manatee sequences were both highly effective in retrieving sequences from H. gigas (mean=98.8% coverage), as were more divergent probes for regions of ENAM (99.0% coverage) that were designed exclusively from a proboscidean (African elephant) and a hyracoid (Cape hyrax). New sequences were combined with available sequences for representatives of all other afrotherian orders. We also expanded a previously published morphological matrix for living and fossil Sirenia by adding both new taxa and nine new postcranial characters. Maximum likelihood and parsimony analyses of the molecular data provide robust support for an association of H. gigas and D. dugon to the exclusion of living trichechids (manatees). Parsimony analyses of the morphological data also support the inclusion of H. gigas in Dugongidae with D. dugon and fossil dugongids. Timetree analyses based on calibration density approaches with hard- and soft-bounded constraints suggest that H. gigas and D. dugon diverged in the Oligocene and that crown sirenians last shared a common ancestor in the Eocene. The coding sequence for the ENAM gene in H. gigas does not contain frameshift mutations or stop codons, but there is a transversion mutation (AG to CG) in the acceptor splice site of intron 2. This disruption in the edentulous Stellers sea cow is consistent with previous studies that have documented inactivating mutations in tooth-specific loci of a variety of edentulous and enamelless vertebrates including birds, turtles, aardvarks, pangolins, xenarthrans, and baleen whales. Further, branch-site dN/dS analyses provide evidence for positive selection in ENAM on the stem dugongid branch where extensive tooth reduction occurred, followed by neutral evolution on the Hydrodamalis branch. Finally, we present a synthetic evolutionary tree for living and fossil sirenians showing several key innovations in the history of this clade including character state changes that parallel those that occurred in the evolutionary history of cetaceans.

Bohaskaia monodontoides, a new monodontid (Cetacea, Odontoceti, Delphinoidea) from the Pliocene of the western North Atlantic Ocean

ABSTRACT Here we describe Bohaskaia monodontoides, a new taxon of beluga-like odontocete cetacean from the early Pliocene Yorktown Formation of Virginia and North Carolina. Among odontocetes, Bohaskaia shares key characteristics of the rostrum and face with belugas (Delphinapterus leucas), narwhals (Monodon monoceros), and Denebola brachycephala from the late Miocene of Baja California, thus placing it as a member of the Monodontidae. It also displays autapomorphies that merit its placement in a new genus and species. Both Denebola and Bohaskaia occurred in warmer latitudes than those of extant monodontids, even accounting for extralimital records. Such data from the fossil record of Monodontidae indicates that putative cold climate adaptations of living monodontids might have appeared under different environmental conditions and that their Holarctic and sub-Arctic distribution is a relatively recent phenomenon.

A new fossil sirenian (Mammalia, Dugonginae) from the miocene of India

Kutchisiren cylindrica, n. gen. n. sp., is a dugongine sirenian represented by a single skull with mandible from the Lower Miocene (Aquitanian or Burdigalian) Khari Nadi Formation in the district of Kutch (= Kachchh), Gujarat State, western India. It is characterized by a braincase of nearly cylindrical form, a strong rostral deflection of about 78°, and a large upper incisor tusk with a lozenge-shaped cross section. It evidently coexisted with at least two other dugongine taxa: Bharatisiren kachchhensis and Domningia sodhae. These three forms differed in rostral deflection, body size, and other details of skull and tusk morphology that apparently enabled them to partition the available marine seagrass resources. Their exact phylogenetic relationships, however, remain unresolved for now.

The earliest Caribbean rodents: Oligocene caviomorphs from Puerto Rico

ABSTRACT The Greater Antilles was once home to a diverse array of endemic groups of caviomorph rodents. Many of these became extinct, together with endemic insectivores, sloths, and primates, in what is widely considered an archtypical example of Quaternary extinction on oceanic islands. When and how the ancestors of these land mammals arrived on these islands is debated. Here we report on the discovery, for the first time, of undoubted rodent incisors in Oligocene contexts in Puerto Rico. The enamel microstructure of the Puerto Rican fossils is consistent with caviomorph affinities, particularly with extinct Pleistocene species from Puerto Rico and Anguilla (Lesser Antilles). These fossils represent the oldest rodents now known from the West Indies, as well as the oldest record of caviomorphs north of the Equator. Their presence in the Greater Antilles by the early Oligocene suggests that caviomorph rodents must have begun to disperse throughout South America, and beyond, very soon after their initial radiation (currently pegged to middle or late Eocene). This, together with other lines of evidence, is consistent with the landspan hypothesis for the origin of the Greater Antillean land mammal fauna, although other mechanisms, including overwater transport, cannot be ruled out.

Eotaria citrica, sp. nov., a new stem otariid from the “Topanga” formation of Southern California

A new taxon of stem otariid, Eotaria citrica sp. nov., is described from the upper Burdigalian to lower Langhian “Topanga” formation of Orange County, California. The new species is described from mandibular and dental remains that show a unique combination of plesiomorphic and derived characters. Specifically, it is characterized by having trenchant and prominent paraconid cusps in p3–m1, lingual cingula of p2–4 with faint crenulations, premolars and molars with vestigial metaconid, bilobed root of m2 and a genial tuberosity located under p3. Furthermore, additional material of the contemporaneous Eotaria crypta is described, providing new information on the morphology of this taxon. Both species of Eotaria represent the earliest stem otariids, reinforcing the hypothesis that the group originated in the north Eastern Pacific Region. At present, the “Topanga” Fm. pinniped fauna includes Eotaria citrica, Eotaria crypta, the desmatophocid Allodesmus sp., the odobenids Neotherium sp., Pelagiarctos sp. and includes the oldest records of crown pinnipeds in California. Overall this pinniped fauna is similar to the nearly contemporaneous Sharktooth Hill bonebed. However, unambiguous records of Eotaria are still missing from Sharktooth Hill. This absence may be due to taphonomic or paleoenvironmental factors. The new “Topanga” record presented here was integrated into an overview of the late Oligocene through early Pleistocene pinniped faunas of Southern California. The results show an overall increase in body size over time until the Pleistocene. Furthermore, desmatophocids were the largest pinnipeds during the middle Miocene, but were extinct by the beginning of the late Miocene. Odobenids diversified and became the dominant pinnipeds in late Miocene through Pleistocene assemblages, usually approaching or exceeding 3 m in body length, while otariids remained as the smallest taxa. This pattern contrasts with modern assemblages, in which the phocid Mirounga angustirostris is the largest pinniped taxon in the region, odobenids are extinct and medium and small size ranges are occupied by otariids or other phocids.

Pygmy Sperm Whales (Odontoceti, Kogiidae) from the Pliocene of Florida and North Carolina

ABSTRACT We describe fossil kogiid periotics from the Lower Pliocene upper Bone Valley Formation in central Florida and the Lower to Upper Pliocene Yorktown Formation at Lee Creek Mine, North Carolina. The fossils show diagnostic characters that identify them as belonging to Kogiidae, such as three spines in the anterior process, presence of an incudal process, and a posterior process oriented along the long axis of the bone. Morphological comparisons and morphometric and statistical analyses of periotic proportions confirm the presence of a large and a small morphotype within the sample. The large morphotype (mean length = 39.76 mm) belongs to an unknown kogiid that occurs in both formations, whereas the small morphotype (mean length = 28.64 mm), referred to aff. Kogia sp., occurs only in the Yorktown Formation. The cooccurrence of two taxa in North Carolina may represent one of the earliest evidences of sympatry in kogiids and may demonstrate that this ecological behavior has been part of the natural history of this group at least since the deposition of the Yorktown Formation at Lee Creek (∼4.8–3.1 Ma). In addition, the occurrence of the large morphotype in the upper Bone Valley Formation, herein reported for the first time, shows that we are still far from understanding the diversity of marine mammals of that formation and that revision of newly acquired material is necessary.