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

Evolutionary Patterns among Living and Fossil Kogiid Sperm Whales: Evidence from the Neogene of Central America

<div><p>Kogiids are known by two living species, the pygmy and dwarf sperm whale (<i>Kogia breviceps</i> and <i>K</i>. <i>sima</i>). Both are relatively rare, and as their names suggest, they are closely related to the sperm whale, all being characterized by the presence of a spermaceti organ. However, this organ is much reduced in kogiids and may have become functionally different. Here we describe a fossil kogiid from the late Miocene of Panama and we explore the evolutionary history of the group with special attention to this evolutionary reduction. The fossil consists of cranial material from the late Tortonian (~7.5 Ma) Piña facies of the Chagres Formation in Panama. Detailed comparison with other fossil and extant kogiids and the results of a phylogenetic analysis place the Panamanian kogiid, herein named <i>Nanokogia isthmia</i> gen. et sp. nov., as a taxon most closely related to <i>Praekogia cedrosensis</i> from the Messinian (~6 Ma) of Baja California and to <i>Kogia</i> spp. Furthermore our results show that reduction of the spermaceti organ has occurred iteratively in kogiids, once in <i>Thalassocetus antwerpiensis</i> in the early-middle Miocene, and more recently in <i>Kogia</i> spp. Additionally, we estimate the divergence between extant species of <i>Kogia</i> at around the late Pliocene, later than previously predicted by molecular estimates. Finally, comparison of <i>Nanokogia</i> with the coeval <i>Scaphokogia cochlearis</i> from Peru shows that these two species display a greater morphological disparity between them than that observed between the extant members of the group. We hypothesize that this reflects differences in feeding ecologies of the two species, with <i>Nanokogia</i> being more similar to extant <i>Kogia</i>. <i>Nanokogia</i> shows that kogiids have been part of the Neotropical marine mammal communities at least since the late Miocene, and gives us insight into the evolutionary history and origins of one of the rarest groups of living whales.</p></div

Ventral views of kogiid skulls.

<p><i>Aprixokogia kelloggi</i> (USNM 187015), 12A, <i>Scaphokogia cochlearis</i> (MNHN PPI 229), 12B, <i>Praekogia cedrosensis</i> (UCMP 315229), 12C, <i>Nanokogia isthmia</i> gen. et sp. nov. (UF 273554), 12D, and (UF 280000), 12E, <i>Kogia sima</i> (LACM 47142), 12F, and <i>K</i>. <i>breviceps</i> (LACM 95745), 12G. Each bone is color-coded for ease of comparison. Areas in white are reconstructed, light gray areas are covered with sediment; diagonal lines denote broken surfaces. Illustrations based on direct observations of the specimens listed.</p

Dorsal, 4A-B, and ventral, 4C-D, views of referred specimen of <i>Nanokogia isthmia</i> gen. et sp. nov. (UF 273554).

<p>Abbreviations: amc, anterior meatal crest; as, alisphenoid; bo, basioccipital; boc, basioccipital crest; dif, dorsal infraorbital foramen; eam, external auditory meatus; en, external nares; fg, frontal groove; fo, foramen ovale; fp, falciform process of squamosal; fr, frontal; jf, jugular foramen; jn, jugular notch; la+j, lacrimal + jugal; lmc, lateral maxillary crest; mx, maxilla; oc, occipital condyles; pmx, premaxilla; pop, postorbital process; prp, preorbital process; prs, presphenoid; ptdl, dorsal lamina of pterygoid; ptml, medial lamina of pterygoid; ptsf, pterygoid sinus fossa; scb, supracranial basin; so, supraoccipital; sq, squamosal; tsr, tympanosquamosal recess; vif, ventral infraorbital foramen; vo, vomer. Gray shaded areas indicate sediment; diagonal lines denote broken surfaces.</p

Right lateral view of holotype skull of <i>Nanokogia isthmia</i> gen. et sp. nov. (UF 280000).

<p>Abbreviations: eo, exoccipital; fg, frontal groove; fr, frontal; la+j, lacrimal + jugal; ma, mandible; mx, maxilla; lmc, lateral maxillary crest; npp, notch for posterior process of tympanic; of, optic foramen; pa, parietal; pgp, postglenoid process; pmx, premaxilla; ptdl, dorsal lamina of pterygoid; ptha, pterygoid hamulus; ptml, medial lamina of pterygoid; ptn, pterygoid notch; ptsf, pterygoid sinus fossa; scb, supracranial basin; sfc, sagittal facial crest; sq, squamosal; tc, temporal crest; zp, zygomatic process. Gray shaded areas indicate sediment; diagonal lines denote broken surfaces.</p

Left, 6A-B, and right, 6C-D, lateral views of referred specimen of <i>Nanokogia isthmia</i> gen. et sp. nov. (UF 273554).

<p>Abbreviations: boc, basioccipital crest; eam, external auditory meatus; eo, exoccipital; fr, frontal; lmc, lateral maxillary crest; mx, maxilla; npp, notch for posterior process of tympanic; oc, occipital condyle; of, optic foramen; pa, parietal; pmx, premaxilla; pop, postorbital process; prp, preorbital process; prs, presphenoid; ptdl, dorsal lamina of pterygoid; ptml, medial lamina of pterygoid; scb, supracranial basin; smc, supramastoid crest; sq, squamosal; tc, temporal crest; vo, vomer; zp, zygomatic process. Gray shaded areas indicate sediment; diagonal lines denote broken surfaces.</p

Right lateral views of kogiid skulls.

<p><i>Aprixokogia kelloggi</i> (left side, reversed, USNM 187015), 13A, <i>Scaphokogia cochlearis</i> (MNHN PPI 229), 13B, <i>Praekogia cedrosensis</i> (UCMP 315229), 13C, <i>Nanokogia isthmia</i> gen. et sp. nov. (based on UF 280000 and 273554), 13D, <i>Kogia sima</i> (LACM 47142), 13E, and <i>K</i>. <i>breviceps</i> (LACM 95745), 13F. Each bone is color-coded for ease of comparison. Areas in white are reconstructed; diagonal lines denote broken surfaces. Illustrations based on direct observations of the specimens listed.</p

Ventral view of holotype skull of <i>Nanokogia isthmia</i> gen. et sp. nov. (UF 280000).

<p>Abbreviations: an, antorbital notch; bo/bs, basioccipital/basisphenoid; boc, basioccipital crest; cb.e, cerebral endocast; eam, external auditory meatus; fg, frontal groove; in, internal nares; jn, jugular notch; la+j, lacrimal + jugal; ma, mandible; mx, maxilla; npp, notch for posterior process of tympanic; oc, occipital condyles; pl, palatine; prs, presphenoid; ptdl, dorsal lamina of pterygoid; ptha, pterygoid hamulus; ptml, medial lamina of pterygoid; ptsf, pterygoid sinus fossa; tsr, tympanosquamosal recess; vif, ventral infraorbital foramen; vo, vomer. Gray shaded areas indicate sediment; diagonal lines denote broken surfaces.</p

Dorsal view of holotype skull of <i>Nanokogia isthmia</i> gen. et sp. nov. (UF 280000).

<p>Abbreviations: adif, anterior dorsal infraorbital foramen; an, antorbital notch; en, external nares; et, ethmoid; fr, frontal; la+j, lacrimal + jugal; lmc, lateral maxillary crest; mrg, mesorostral groove; mx, maxilla; pmx, premaxilla; pdif, posterior dorsal infraorbital foramen; scf, supracranial fossa; sfc, sagittal facial crest; sq, squamosal; vo, vomer. Gray shaded areas indicate sediment; diagonal lines denote broken surfaces. Gray shaded areas indicate sediment; diagonal lines denote broken surfaces.</p