Rachel A. Racicot

The Pliocene marine mammal fauna is the best sampled and most intensely studied in the world, preserving many modern genera of cetaceans. Today, harbor porpoises ( Phocoena phocoena Linnaeus, 1758) as well as other extant genera of odontocetes ( Tursiops , Stenella , Lagenorhynchus , Physeter , Pseudorca , Globicephala ) inhabit the western North Atlantic. Many similar forms were reported from the early Pliocene Yorktown Formation—except for porpoises (Phocoenidae), which are key members of modern marine ecosystems, but surprisingly absent from the fossil record in this region. Hence, there are unresolved questions concerning the dispersal and biogeography of this group. In this study, we report the first fossil Phocoenidae from the western North Atlantic, a series of four isolated periotics of Phocoena sp. from Pliocene strata near Charleston, South Carolina, USA, and describe the bony labyrinth of one of these. The periotics show several external anatomical characters diagnostic for phocoenids, and we used μCT scanning to reconstruct a three-dimensional model of the bony labyrinth to further test this interpretation. We performed 13 measurements on the model surface and compared those with a sampling of other phocoenid specimens. By assessing the taxonomic affinity of the fossils, we gain a better understanding of marine ecosystems through a dynamic interval of the Neogene and shed light on the origins and dispersal of a major odontocete clade.

The “second wave” of Ediacaran evolution (∼558–548 Ma) was characterized by the appearance of macroscopic organisms in shallow marine settings, where they formed communities with high morphological and ecological diversity, including new and more complex modes of life. Based on analogy with modern marine ecosystems, these early shallow water communities could have substantially modified local hydrodynamic conditions and influenced resource availability, but we know very little about how they interacted with their fluid environment at larger spatial scales. Here, we use computational fluid dynamics to investigate the hydrodynamics of different shallow marine Ediacaran communities based on fossil surfaces from Russia and South Australia. Our results reveal considerable hydrodynamic variability among these communities, ranging from unobstructed flow, to enhanced mixing, to very low in-canopy flow. This variability represents a noticeable shift from the more conserved hydrodynamic conditions reconstructed for older Ediacaran communities from deep water settings. The variation in how shallow marine Ediacaran communities affected local hydrodynamics could have given rise to notable differences in the distribution of crucial water-borne resources such as organic carbon and oxygen. We therefore hypothesize that increasing variability in community hydrodynamics was an important source of habitat heterogeneity during the late Ediacaran. On long timescales, this heterogeneity may have helped sculpt ecological opportunity, fostering the radiation of animals.

The Messel Pit is a Konservat-Lagerstätte in Germany, representing the deposits of a latest early to earliest middle Eocene maar lake, and one of the first palaeontological sites to be included on the list of UNESCO World Heritage Sites. One aspect of Messel that makes it so extraordinary is that its sediments are rich in different fossilised organisms – microfossils, plants, fungi, invertebrate animals and vertebrates – that are rarely preserved together. We present an updated list of all taxa, named or not, that have been documented at Messel, comprising 1409 taxa, which represent a smaller but inexactly known number of biological species. The taxonomic list of Labandeira and Dunne (2014) contains serious deficiencies and should not be used uncritically. Furthermore, we compiled specimen lists of all Messel amphibians, reptiles and mammals known to us. In all, our analyses incorporate data from 32 public collections and some 20 private collections. We apply modern biodiversity-theoretic techniques to ascertain how species richness tracks sampling, to estimate what is the minimum asymptotic species richness, and to project how long it will take to sample a given proportion of that minimum richness. Plant and insect diversity is currently less well investigated than vertebrate diversity. Completeness of sampling in aquatic and semiaquatic, followed by volant, vertebrates is higher than in terrestrial vertebrates. Current excavation rates are one-half to two-thirds lower than in the recent past, leading to much higher estimates of the future excavation effort required to sample species richness more completely, should these rates be maintained. Species richness at Messel, which represents a lake within a paratropical forest near the end of the Early Eocene Climate Optimum, was generally higher than in comparable parts of Central Europe today but lower than in present-day Neotropical biotopes. There is no evidence that the Eocene Messel ecosystem was a “tropical rainforest.”

As fully aquatic mammals, hearing is arguably the most important sensory component of cetaceans. Increasingly, researchers have been harnessing computed tomography (CT) to investigate the details of the inner ear as they can provide clues to the hearing abilities of whales. We use microCT scans of a broad sampling of the ear bones (periotics) of primarily toothed whales (Odontoceti) to investigate the inner ear bony labyrinth shape and reconstruct hearing sensitivities among these cetaceans, including several taxa about which little is currently known. We find support for sensitivity to the lower frequency spectrum in the archaeocete Zygorhiza kochii and an early toothed mysticete cf. Aetiocetus. Oligocene odontocetes (including one from our novel dataset), stem delphinidans, and two additional species of the long‐snouted eurhinodelphinids are found to have been able to hear within the narrow‐band high‐frequency spectrum (NBHF), which is thought to be a specialized form of hearing that evolved convergently multiple different times in extant groups to avoid predation by macroraptorial predators. Our results thus indicate that NBHF evolved as early as the Oligocene and certainly in stem delphinidans by the early Miocene, and thus may be an ancestral characteristic rather than a more recent innovation in select groups.

During major evolutionary transitions, groups develop radically new body plans and radiate into new habitats. A classic example is cetaceans which evolved from terrestrial ancestors to become pelagic swimmers. In doing so, they altered their air-filled sinuses, transitioning some of these spaces to allow for fluctuations in air capacity and storage via soft tissue borders. Other tetrapods independently underwent land-to-sea transitions, but it is unclear if they similarly changed their sinuses. We use computed tomography to study sinus changes in thalattosuchian crocodylomorphs that transformed from land-bound ancestors to become the only known aquatic swimming archosaurs. We find that thalattosuchian braincase sinuses reduced over their transition, similar to cetaceans, but their snout sinuses counterintuitively expanded, distinct from cetaceans, and that both trends were underpinned by high evolutionary rates. We hypothesize that aquatic thalattosuchians were ill suited to deep diving by their snout sinuses, which seem to have remained large to help drain their unusual salt glands. Thus, although convergent in general terms, thalattosuchians and cetaceans were subject to different constraints that shaped their transitions to water. Thalattosuchians attained a stage similar to less pelagic transitional forms in the cetacean lineage (late protocetid-basilosaurid) but did not become further specialized for ocean life.

Pteridinium simplex is an iconic erniettomorph taxon best known from late Ediacaran successions in South Australia, Russia, and Namibia. Despite nearly 100 years of study, there remain fundamental questions surrounding the paleobiology and paleoecology of this organism, including its life position relative to the sediment–water interface, and how it fed and functioned within benthic communities. Here, we combine a redescription of specimens housed at the Senckenberg Forschungsinstitut und Naturmuseum Frankfurt with field observations of fossiliferous surfaces, to constrain the life habit of Pteridinium and gain insights into the character of benthic ecosystems shortly before the beginning of the Cambrian. We present paleontological and sedimentological evidence suggesting that Pteridinium was semi-infaunal and lived gregariously in aggregated communities, preferentially adopting an orientation with the long axis perpendicular to the prevailing current direction. Using computational fluid dynamics simulations, we demonstrate that this life habit could plausibly have led to suspended food particles settling within the organism's central cavity. This supports interpretation of Pteridinium as a macroscopic suspension feeder that functioned similarly to the coeval erniettomorph Ernietta , emblematic of a broader paleoecological shift toward benthic suspension-feeding strategies over the course of the latest Ediacaran. Finally, we discuss how this new reconstruction of Pteridinium provides information concerning its potential relationships with extant animal groups and state a case for reconstructing Pteridinium as a colonial metazoan.

Studies surrounding the evolution of sensory system anatomy in cetaceans over the last ~100 years have shed light on aspects of the early evolution of hearing sensitivities, the small relative size of the organ of balance (semicircular canals and vestibule), brain (endocast) shape and relative volume changes, and ontogenetic development of sensory‐related structures. Here, I review advances in our knowledge of sensory system anatomy as informed by the use of nondestructive imaging techniques, with a focus on applied methods in computed tomography (CT and μCT), and identify the key questions that remain to be addressed. Of these, the most important are: Is lower frequency hearing sensitivity the ancestral condition for whales? Did echolocation evolve more than once in odontocetes; and if so, when and why? How has the structure of the cetacean brain changed, through the evolution of whales, and does this correspond to changes in hearing sensitivities? Finally, what are the general pathways of ontogenetic development of sensory systems in odontocetes and mysticetes? Answering these questions will allow us to understand important macroevolutionary patterns in a fully aquatic mammalian group and provides baseline data on species for which we have limited biological information because of logistical limitations.

Bony labyrinth morphology varies across marine mammals and contains key information regarding hearing sensitivity and ecology. The hearing ranges of globicephaline (Delphinidae: Globicephalinae) or melon‐headed dolphins, known as “Blackfish,” have been extensively studied using acoustic technologies, but clade‐wide morphological analysis of the bony labyrinth is lacking. In this study, we investigate the variation in hearing‐relevant bony labyrinth morphology within globicephalines using μCT scans of isolated petrosals and digitally isolating the bony labyrinth of all species. Principal components analysis (PCA) of nine hearing‐relevant measurements of the cochlea alongside a broader sampling of terrestrial and aquatic artiodactyls shows Orcaella brevirostris and other globicephalines with higher levels of facial asymmetry and potentially more specialized echolocation abilities plotting near Monodon monoceros, Delphinapterus leucas, and Orcinus orca. The remaining globicephalines, which have more symmetrical skulls and other unique and acoustically relevant attributes, plotted towards the middle of the echolocating odontocete portion of the PCA. Our analysis thus reveals that inner ear morphology may correlate with both facial skull morphology and echolocation specializations, as these are intertwined. Furthermore, this study illustrates how morphological analyses, especially those centered on hearing, may provide critical conservation‐relevant information as direct access to audiograms becomes less tenable.