Natalia Rybczynski

Significantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies

Temperatures in the Arctic have increased by an astounding 1 °C in response to anthropogenic forcing over the past 20 years and are expected to rise further in the coming decades. The Pliocene (2.6–5.3 Ma) is of particular interest as an analog for future warming because global temperatures were signifi cantly warmer than today for a sustained period of time, with continental confi gurations similar to present. Here, we estimate mean annual temperature (MAT) based upon three independent proxies from an early Pliocene peat deposit in the Canadian High Arctic. Our proxies, including oxygen isotopes and annual ring widths (MAT = –0.5 ± 1.9 °C), coexistence of paleovegetation (MAT = –0.4 ± 4.1 °C), and bacterial tetraether composition in paleosols (MAT = –0.6 ± 5.0 °C), yield estimates that are statistically indistinguishable. The consensus among these proxies suggests that Arctic temperatures were ~19 °C warmer during the Pliocene than at present, while atmospheric CO 2 concentrations were ~390 ppmv. These elevated Arctic Pliocene temperatures result in a greatly reduced and asymmetrical latitudinal temperature gradient that is probably the result of increased poleward heat transport and decreased albedo. These results indicate that Arctic temperatures may be exceedingly sensitive to anthropogenic CO 2 emissions.

A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia

Modern pinnipeds (seals, sea lions and the walrus) are semi-aquatic, generally marine carnivores the limbs of which have been modified into flippers. Recent phylogenetic studies using morphological and molecular evidence support pinniped monophyly, and suggest a sister relationship with ursoids (for example bears) or musteloids (the clade that includes skunks, badgers, weasels and otters). Although the position of pinnipeds within modern carnivores appears moderately well resolved, fossil evidence of the morphological steps leading from a terrestrial ancestor to the modern marine forms has been weak or contentious. The earliest well-represented fossil pinniped is Enaliarctos, a marine form with flippers, which had appeared on the northwestern shores of North America by the early Miocene epoch. Here we report the discovery of a nearly complete skeleton of a new semi-aquatic carnivore from an early Miocene lake deposit in Nunavut, Canada, that represents a morphological link in early pinniped evolution. The new taxon retains a long tail and the proportions of its fore- and hindlimbs are more similar to those of modern terrestrial carnivores than to modern pinnipeds. Morphological traits indicative of semi-aquatic adaptation include a forelimb with a prominent deltopectoral ridge on the humerus, a posterodorsally expanded scapula, a pelvis with relatively short ilium, a shortened femur and flattened phalanges, suggestive of webbing. The new fossil shows evidence of pinniped affinities and similarities to the early Oligocene Amphicticeps from Asia and the late Oligocene and Miocene Potamotherium from Europe. The discovery suggests that the evolution of pinnipeds included a freshwater transitional phase, and may support the hypothesis that the Arctic was an early centre of pinniped evolution.

Earliest evidence for efficient oral processing in a terrestrial herbivore

Herbivores can increase their digestion rate by mechanically reducing particle size through oral trituration. Groups of terrestrial vertebrates with the greatest capacity to reduce tough plant foods orally are also the most abundant and diverse, as exemplified by ornithopod dinosaurs during the Mesozoic and extant artiodactyl and perissodactyl mammals. Thus, the effective oral processing of high-fibre plant material seems to represent an evolutionary innovation of both functional and macroevolutionary significance. However, evidence for oral processing is poorly documented in the fossil record, especially during the initial stages of terrestrial vertebrate diversification. Here we report on the basal anomodont Suminia getmanovi, the only known Palaeozoic vertebrate in which unequivocal specializations in its cranium and teeth for high-fibre herbivory are well preserved. We propose that the capacity to comminute tough plant foods was critical to the diversification of anomodonts, the most diverse, widely dispersed and abundant group of Palaeozoic terrestrial vertebrates, and to the onset of modern terrestrial ecosystems.

A NEW, UNUSUAL CASTORID (RODENTIA) FROM THE EARLIEST MIOCENE OF NEBRASKA

Abstract Migmacastor procumbodens, gen. et sp. nov. is described from the late Arikareean of Nebraska based on a nearly complete skull. The new taxon exhibits various incisal specializations (e.g., incisors elongated along longitudinal axis, procumbent upper incisors) that are associated with tooth-digging behavior in modern rodents. Migmacastor does not belong in the Palaeocastorinae, the previously recognized fossorial clade of beavers, nor in any other castorid subfamily. Phylogenetic analysis indicates it is more derived than Agnotocastorinae and is a sister taxon to a monophyletic clade of beavers composed of both fossorial and semiaquatic taxa (Castorinae + Palaeocastorinae + Castoroidinae). Migmacastor is united to the latter unnamed clade by five unambiguous characters: lack of P3; upper tooth rows that diverge posteriorly; P4 larger than molars; smooth enamel on cheek teeth; and longitudinally grooved palate. Preliminary phylogenetic and morphological evidence suggest that tooth-digging behavior arose at least twice in North American beavers.

Woodcutting behavior in beavers (Castoridae, Rodentia): estimating ecological performance in a modern and a fossil taxon

Abstract An early Pliocene fossil locality in the Canadian High Arctic preserves the remains of the extinct beaver Dipoides sp. (Castoridae, Rodentia) in association with an assemblage of fossil beaver-cut wood. The wood assemblage presents a first opportunity to investigate woodcutting behavior and ecological performance in an extinct castorid genus. This study compares woodcutting in Dipoides sp. with that of the modern beaver, Castor canadensis, using evidence from small-diameter cut sticks (i.e., sticks transected by parallel series of cut marks) in combination with behavioral observations of Castor woodcutting. During woodcutting both Castor and Dipoides used their incisors unilaterally; the upper incisor was pressed against the stick while the corresponding lower incisor cut. Cut marks were relatively larger for Castor than Dipoides (scaled to incisor size). Compared with Dipoides, Castor more frequently used a cutting strategy that minimized the number of cuts needed to transect a stick (e.g., clipping as opposed to chip removal). Taken together, the behavioral evidence suggests that ecological cutting performance was lower for Dipoides than Castor.

Early Eocene Mammals from the Driftwood Creek Beds, Driftwood Canyon Provincial Park, Northern British Columbia

ABSTRACT The early Eocene is an important time in Cenozoic history because it marked the height of global warming, coincident with significant reorganization of the mammalian biota. In North America, our understanding of mammalian diversity during this interval is largely limited to a fossil record south of the 49th Parallel. New discoveries in the early Eocene Driftwood Creek beds (Ootsa Lake Group), northern British Columbia (∼55°N) double the known diversity of Eocene mammals from this Canadian province and provide a window into the mammalian community that lived near the northernmost lake of the Okanagan Highlands, a series of Eocene lake deposits extending north-south from Republic, Washington, to Smithers, northern British Columbia. A diverse insect and fish fauna has been described from Okanagan Highlands Eocene lake shales, together with a diverse flora, interpreted as a cool upland forested landscape. We report the tapiroid cf. Heptodon and an erinaceomorph lipotyphlan Silvacola acares, gen. et sp. nov., from the Driftwood Creek beds. Presence of cf. Heptodon is consistent with the late early Eocene age of the Driftwood Creek beds determined by radiometric dating and palynology. Heptodon is otherwise known from Eocene localities in Wyoming and Colorado as well as Ellesmere Island in the High Arctic, whereas erinaceids are recorded from late Paleocene sites in Alberta, Saskatchewan, and the U.S. Western Interior and are relatively uncommon at Eocene sites in the U.S. Western Interior. Occurrence of cf. Heptodon at Driftwood Canyon supports the hypothesis proposed by others that tapiroids are proxies of densely forested habitats.

Re-Evaluation of Sinocastor (Rodentia: Castoridae) with Implications on the Origin of Modern Beavers

The extant beaver, Castor, has played an important role shaping landscapes and ecosystems in Eurasia and North America, yet the origins and early evolution of this lineage remain poorly understood. Here we use a geometric morphometric approach to help re-evaluate the phylogenetic affinities of a fossil skull from the Late Miocene of China. This specimen was originally considered Sinocastor, and later transferred to Castor. The aim of this study was to determine whether this form is an early member of Castor, or if it represents a lineage outside of Castor. The specimen was compared to 38 specimens of modern Castor (both C. canadensis and C. fiber) as well as fossil specimens of C. fiber (Pleistocene), C. californicus (Pliocene) and the early castorids Steneofiber eseri (early Miocene). The results show that the specimen falls outside the Castor morphospace and that compared to Castor, Sinocastor possesses a: 1) narrower post-orbital constriction, 2) anteroposteriorly shortened basioccipital depression, 3) shortened incisive foramen, 4) more posteriorly located palatine foramen, 5) longer rostrum, and 6) longer braincase. Also the specimen shows a much shallower basiocciptal depression than what is seen in living Castor, as well as prominently rooted molars. We conclude that Sinocastor is a valid genus. Given the prevalence of apparently primitive traits, Sinocastor might be a near relative of the lineage that gave rise to Castor, implying a possible Asiatic origin for Castor.