Brian P. Kraatz

Eocene–Oligocene transition in Central Asia and its effects on mammalian evolution

The Eocene-Oligocene boundary (EOB) marks a period of dramatic global climatic change correlated with pronounced mammalian faunal change. The timing of these events is well constrained in North America and Europe, but the Asian record has yet to produce a synthetic section linking environmental change, mammalian fossils, and precise geochronological dates. Here we present the first magnetostratigraphic section for the Hsanda Gol Formation, Mongolia, which yields significant Oligocene fossils and also marks a pattern of aridification that is tightly correlated to the EOB (33.9 Ma), supporting a broader pattern of aridification in the central Asian plateau across the EOB. Oligocene faunas of Asia can now be confidently correlated to those of North America, Europe, and Africa. These results suggest that mammalian faunal turnover within Asia occurred slightly later than similar events within Europe, and question the influence of Asian immigrants on the Grande Coupure faunal turnover.

Early evidence for complex social structure in Proboscidea from a late Miocene trackway site in the United Arab Emirates

Many living vertebrates exhibit complex social structures, evidence for the antiquity of which is limited to rare and exceptional fossil finds. Living elephants possess a characteristic social structure that is sex-segregated and multi-tiered, centred around a matriarchal family and solitary or loosely associated groups of adult males. Although the fossil record of Proboscidea is extensive, the origin and evolution of social structure in this clade is virtually unknown. Here, we present imagery and analyses of an extensive late Miocene fossil trackway site from the United Arab Emirates. The site of Mleisa 1 preserves exceptionally long trackways of a herd of at least 13 individuals of varying size transected by that of a single large individual, indicating the presence of both herding and solitary social modes. Trackway stride lengths and resulting body mass estimates indicate that the solitary individual was also the largest and therefore most likely a male. Sexual determination for the herd is equivocal, but the body size profile and number of individuals are commensurate with those of a modern elephant family unit. The Mleisa 1 trackways provide direct evidence for the antiquity of characteristic and complex social structure in Proboscidea.

A New Species of Gomphos (Glires, Mammalia) from the Eocene of the Erlian Basin, Nei Mongol, China

Dental and postcranial specimens of Gomphos shevyrevae, sp. nov., from the lower part of the Irdin Manha Formation at the Huheboerhe locality, Erlian Basin, Nei Mongol (Inner Mongolia), are described. The new species differs from G. elkema and G. ellae in having more robust teeth with inflated cusps and stronger lophs and a calcaneus with extra articulation for the astragalus and navicular. The new species is stratigraphically well constrained and probably represents the youngest known species of the genus, extending its geological record into the Middle Eocene. It also shows that mimotonids coexisted for millions of years as a side branch of duplicidentates with the earliest stem lagomorphs, including Dawsonolagus.

Response to comment on "The placental mammal ancestor and the post-K-Pg radiation of placentals".

Tree-building with diverse data maximizes explanatory power. Application of molecular clock models to ancient speciation events risks a bias against detection of fast radiations subsequent to the Cretaceous-Paleogene (K-Pg) event. Contrary to Springer et al., post–K-Pg placental diversification does not require “virus-like” substitution rates. Even constraining clade ages to their model, the explosive model best explains placental evolution.

Diversity in the later Paleogene proboscidean radiation: a small barytheriid from the Oligocene of Dhofar Governorate, Sultanate of Oman

Despite significant recent improvements to our understanding of the early evolution of the Order Proboscidea (elephants and their extinct relatives), geographic sampling of the group’s Paleogene fossil record remains strongly biased, with the first ~30 million years of proboscidean evolution documented solely in near-coastal deposits of northern Africa. The considerable morphological disparity that is observable among the late Eocene and early Oligocene proboscideans of northern Africa suggests that other, as yet unsampled, parts of Afro-Arabia might have served as important centers for the early diversification of major proboscidean clades. Here we describe the oldest taxonomically diagnostic remains of a fossil proboscidean from the Arabian Peninsula, a partial mandible of Omanitherium dhofarensis (new genus and species), from near the base of the early Oligocene Shizar Member of the Ashawq Formation, in the Dhofar Governorate of the Sultanate of Oman. The molars and premolars of Omanitherium are morphologically intermediate between those of Arcanotherium and Barytherium from northern Africa, but its specialized lower incisors are unlike those of other known Paleogene proboscideans in being greatly enlarged, high-crowned, conical, and tusk-like. Omanitherium is consistently placed close to late Eocene Barytherium in our phylogenetic analyses, and we place the new genus in the Family Barytheriidae. Some features of Omanitherium, such as tusk-like lower second incisors, the possible loss of the lower central incisors, an enlarged anterior mental foramen, and inferred elongate mandibular symphysis and diminutive P2, suggest a possible phylogenetic link with Deinotheriidae, an extinct family of proboscideans whose origins have long been mysterious.

Evolutionary Patterns in the Dentition of Duplicidentata (Mammalia) and a Novel Trend in the Molarization of Premolars

Background The cusp homology of Lagomorpha has long been problematic largely because their teeth are highly derived relative to their more typically tribosphenic ancestors. Within this context, the lagomorph central cusp has been particularly difficult to homologize with other tribosphenic cusps; authors have previously considered it the paracone, protocone, metacone, amphicone, or an entirely new cusp. Methodology/Principal Findings Here we present newly described fossil duplicidentates (Lagomorpha and Mimotonidae) in the context of a well-constrained phylogeny to establish a nomenclatural system for cusps based on the tribosphenic pattern. We show that the central cusp of lagomorphs is homologous with the metaconule of other mammals. We also show that the buccal acquisition of a second cusp on the premolars (molarization) within duplicidentates is atypical with respect to other mammalian lineages; within the earliest lagomorphs, a second buccal cusp is added mesially to an isolated buccal cusp. Conclusions/Significance The distal shift of the ‘ancestral’ paracone within early duplicidentates amounts to the changing of a paracone into a metacone in these lineages. For this reason, we support a strictly topological approach to cusp names, and suggest a discontinuity in nomenclature to capture the complexity of the interplay between evolutionary history and the developmental process that have produced cusp patterns in duplicidentates.

Central Asian aridification during the late Eocene to early Miocene inferred from preliminary study of shallow marine-eolian sedimentary rocks from northeastern Tajik Basin

The tempo-spatial development of the Cenozoic Asian aridification across the Eocene-Oligocene and its controlling factors are important scientific topics in Earth Sciences, which are pertinent to regional and global tectonic and climatic events. However, sedimentary rocks preserving the record of aridification during this time from central Asia (ACA) are rare. Here we present a preliminary analysis of sedimentary facies of the lower Paleogene in the northeastern Tajik Basin, which reveals that: the lower part of the studied section is dominated by shallow marine deposits of the Paratethys, the middle part is typical of alternations of eolian dune and fluvial deposits, the upper part is represented by eolian loess-sandy loess (L&SL) like facies, and the top exhibits alternations of fluvial-lacustrine and loess like facies. Based on a chronological framework derived from preliminary magnetostratigraphy, published U-Pb dating of a volcanic ash, and regional litho-stratigraphic correlations, we determine that eolian and L&SL facies accumulated in the northeastern Tajik Basin during the Late Eocene and through most of the Oligocene. These sedimentary units indicate that semi-arid to arid environments of ACA had developed at least since the late Eocene. This initial aridification is closely linked to the westward retreat of the Paratethys that was likely driven by a combination of tectonic activity and sea level changes.

Evolutionary morphology of the rabbit skull.

The skull of leporids (rabbits and hares) is highly transformed, typified by pronounced arching of the dorsal skull and ventral flexion of the facial region (i.e., facial tilt). Previous studies show that locomotor behavior influences aspects of cranial shape in leporids, and here we use an extensive 3D geometric morphometrics dataset to further explore what influences leporid cranial diversity. Facial tilt angle, a trait that strongly correlates with locomotor mode, significantly predicts the cranial shape variation captured by the primary axis of cranial shape space, and describes a small proportion (13.2%) of overall cranial shape variation in the clade. However, locomotor mode does not correlate with overall cranial shape variation in the clade, because there are two district morphologies of generalist species, and saltators and cursorial species have similar morphologies. Cranial shape changes due to phyletic size change (evolutionary allometry) also describes a small proportion (12.5%) of cranial shape variation in the clade, but this is largely driven by the smallest living leporid, the pygmy rabbit (Brachylagus idahoensis). By integrating phylogenetic history with our geometric morphometric data, we show that the leporid cranium exhibits weak phylogenetic signal and substantial homoplasy. Though these results make it difficult to reconstruct what the ‘ancestral’ leporid skull looked like, the fossil records suggest that dorsal arching and facial tilt could have occurred before the origin of the crown group. Lastly, our study highlights the diversity of cranial variation in crown leporids, and highlights a need for additional phylogenetic work that includes stem (fossil) leporids and includes morphological data that captures the transformed morphology of rabbits and hares.

Lagomorph Research: Past, Present, and Future

Too often lagomorphs are overlooked for their evermore-studied cousins, the rodents. This historical pattern is due largely to the perception that a rabbit is simply a rabbit, and that there is a broadly ‘conserved’ biology among extant and extinct species of lagomorphs. This is certainly not the case, and Lagomorph Biology: Evolution, Ecology, and Conservation nicely illustrates that lagomorphs represent a diverse group of mammals rich with tractable research problems and, more importantly, ripe for exposure to new researchers asking new questions. Because this perspective is rare, the publication of this book in itself is a tremendous success for lagomorph researchers. This edited volume is largely the product of the 2nd World Lagomorph Conference held in Vairão, Portugal in 2004. The second conference was long overdue and held 25 years after the first iteration in Canada. Lagomorph Biology marks the current state of lagomorph research and summarizes it in the following sections: Paleontology and Evolution, Population Ecology and Dynamics, Physiology and Behavior, Diseases, and Conservation Management. As the title suggests and the editors note, Lagomorph Biology attempts to be an overview of the broad range of biological research topics that focus on lagomorphs and, for the most part, it is successful. Overall, the papers are of high quality, the book is well edited, and the included illustrations are good. As with any edited volume, however, there is a range of successes in the ability of individual papers to convey intricacies to readers who are often scientifically literate, but may not have the necessary historical background on specific topics. This highlights one of the limits of this book; it does not completely fill the need for a significant publication on lagomorphs that synthesizes a broad range of topics while also serving as an introduction to researchers and students just initiating an interest in lagomorphs. In this sense, there is a missed opportunity to bring scientists up to speed on current research topics in lagomorph biology. The section on disease is a good example where an introductory paper highlighting the basic biology of common viruses as well as an overview of the research history would have been an excellent addition. It must be noted that there are adequate to good overviews of the lagomorph fossil record, pika biology, and conservation of endangered lagomorphs, and these papers set up their respective sections reasonably well. A subtle, but interesting part of the book is a section titled “Trends in Lagomorph Research,” found in the introduction. Here the authors review and compare research topics discussed at the 1979 World Lagomorph Conference to those in 2004. Several prominent trends are recognized; among them are a decrease in studies about the general life history of lagomorphs, and a great increase in taxonomic studies J Mammal Evol (2008) 15:299–300 DOI 10.1007/s10914-008-9084-8