Anatomy of the Nasal and Auditory Regions of the Fossil Lagomorph Palaeolagus haydeni: Systematic and Evolutionary Implications

Palaeolagus, a late Eocene to early Miocene North American lagomorph genus, represented by numerous and well-preserved specimens, has been long considered a basal leporid, although it is currently understood as a stem lagomorph. Based on micro-computed tomography (μCT) data and 3D reconstructions, here we present the first description of intracranial structures of the nasal and auditory regions of a complete skull of Palaeolagus haydeni from the early Oligocene of Nebraska. Although Palaeolagus haydeni shows a puzzling mixture of extant leporid and ochotonid characters, it helps to polarize and re-evaluate already known lagomorph intracranial characters based on outgroup comparison with Rodentia and Scandentia. Common derived features of Palaeolagus haydeni and extant Lagomorpha are the dendritic maxilloturbinal and the excavated nasoturbinal that contacts the lamina semicircularis. Generally, Palaeolagus haydeni and Leporidae have several characters in common, some of which are certainly plesiomorphic (e.g., thin wall of bulla tympani and flat conic cochlea). Palaeolagus haydeni resembles Leporidae in having an interturbinal between the two frontoturbinals, and three ethmoturbinals plus one interturbinal between ethmoturbinal I and II. Now, this should also be regarded as a plesiomorphic grundplan pattern for Leporidae whereas ochotonids are derived from the lagomorph grundplan as concerns the number of frontoturbinals. Concerning the middle ear, Palaeolagus haydeni significantly contributes to the polarization of the anterior anchoring of the malleus in extant lagomorphs. Palaeolagus haydeni resembles the pattern observed in early ontogenetic stages of Ochotonidae, i.e., the attachment of the malleus to the ectotympanic via a short processus anterior. The patterns in adult ochotonids and leporids now can be regarded as two different and apomorphic character states. Autapomorphic characters of Palaeolagus haydeni are the reduced frontoturbinal 2 and the additional anterolaterally oriented process of the lamina semicircularis. Interestingly, among the investigated intracranial structures the loss of the secondary crus commune is the only apomorphic grundplan character of crown Lagomorpha

Chadrolagus and Litolagus.

76 pages : illustrations, map ; 26 cmLagomorphs from the late Eocene and Eocene-Oligocene transition of North America, apart from the abundant and fairly speciose Palaeolagus, are represented by some rare and enigmatic genera, including monospecific Chadrolagus and Litolagus, of uncertain infraordinal relationship. In this paper new specimens of lagomorphs from the Chadronian and Orellan of Montana and Wyoming are presented. They include Chadrolagus emryi from the early Chadronian of the Renova Formation, Beaverhead Basin (Montana), and the late Chadronian of the Dunbar Creek and Cook Ranch Formations (Montana), extending the stratigraphic range of this species to the entire Chadronian interval. Further, previously unreported material, originally collected by M.F. Skinner during his fieldwork in Wyoming (near the Chadronian-Orellan boundary in age), is described and figured. This includes an exquisitely preserved skull of Litolagus molidens, and Limitolagus roosevelti, gen. et sp. nov., represented by mandibular and dental material from the Chadronian-Orellan boundary. L. roosevelti shares many similarities in dental structure with Chadrolagus, but its size is close to Litolagus molidens. The information provided by newly described material and a revision of the type specimens and topotypic series of Chadrolagus emryi and Litolagus molidens allow for an extended diagnosis of these taxa and a preliminary phylogenetic analysis of Paleogene North American lagomorphs with Desmatolagus gobiensis as an outgroup. The skull of Litolagus molidens shows many derived characters: shortening of the palatal bridge and the reduction of its palatine portion, greater skull height, and an increase in the size of the auditory bullae. The greater basicranial angle and relatively wider choanae indicate increased cursorial adaptations. A cladistic analysis shows that there is no immediate ancestral relationship between Chadrolagus and Litolagus, and it supports an advanced phylogenetic position for Litolagus closer to Archaeolaginae, suggested by the cranial and dental morphology. Limitolagus is related closely to Chadrolagus and paraphyly of Palaeolagus is strongly suggested, due to a placement of P. burkei between Limitolagus and the clade composed of Archaeolagus + Litolagus

Auditory region circulation in Lagomorpha: The internal carotid artery pattern revisited

The internal carotid artery (ICA) is one of the major vessels in the cranial circulation. Characters concerning the ICA, such as its course in the auditory region, have been employed frequently in phylogenetic analyses of mammals, including extinct taxa. In lagomorphs, however, our knowledge on vascular features of the auditory region has been based predominantly on living species, mostly on the European rabbit. We present the first survey on 11 out of 12 extant genera and key fossil taxa such as stem lagomorphs and early crown representatives (Archaeolagus and Prolagus). The ICA pattern shows a modified transpromontorial course in stem taxa (Litolagus, Megalagus and Palaeolagus) and Archaeolagus, which we propose as the ancestral character state for Lagomorpha, similar to that for the earliest rodents, plesiadapids and scandentians. The ICA pattern in leporids is perbullar, but shows structural similarities to stem taxa, whereas the extrabullar ICA course in Ochotona is apparently a highly derived condition. Prolagus shows a mixed character state between leporids and Ochotona in its ICA route. The persistence of the transpromontorial ICA course and similarities in the carotid canal structure among stem taxa and crown leporids support morphological conservatism in Lagomorpha, in contrast to their sister clade Rodentia

Bone histology of Protoceratops andrewsi from the Late Cretaceous of Mongolia and its biological implications

Protoceratops andrewsi is one of the best known and abundant ornithischian dinosaurs from the Djadokhta Formation (Late Cretaceous, Mongolia) and a subject of many morphological studies. Here we present the first study of its bone tissue (from the long bones, frill, and rib), describing microstructure, extent of remodeling, and growth tempo changes in ontogeny. Several specimens representing juvenile, subadult, and adult age stages have been studied. In general, paleohistology of Protoceratops is quite uniform throughout ontogeny, showing basic fibrolamellar bone complex with prevalence of woven-fibered bone and scarce remodeling. In adults the parallel-fibered bone matrix forms distinct although irregular zonation in the cortex until dominating it. The bone displays noteworthy abundance of fossilized fibers (including Sharpey’s fibers), which apparently strengthen the tissue and enhance its elasticity. Growth tempo increased in the studied femora of Protoceratops at the subadult stage, which suggests changes in bone proportions (i.e., elongation of the hind limbs) in a similar manner as it was observed in a more basal Psittacosaurus

Small mammal fauna from Wulanhuxiu (Nei Mongol, China) implies the Irdinmanhan–Sharamurunian (Eocene) faunal turnover

Wulanhuxiu, a middle Eocene locality in the Erlian Basin, Nei Mongol (China) has been commonly regarded as belonging to the Ulan Shireh Formation, equated with the Irdin Manha Formation. We recognized two separate mammalian faunas of different age from the beds exposed at Wulanhuxiu. The lower fossiliferous horizon contains an anagalid, uncommon duplicidentate representatives (Gomphos progressus sp. nov., Mimolagus, Erenlagus, and Strenulagus), and diverse perissodactyls. This combination of taxa points to an Irdinmanhan age, but one element of the fauna (Schlosseria) may represent an Arshantan relic. Overall, the assemblage comprises “paleoplacental” mammals mixed with “neoplacentals”. The upper horizon is less species-rich and the only paleoplacentals present are scarce creodonts. However, this horizon is marked by abundant remains (including postcranial material) of the lagomorph Gobiolagus and by the presence of an advanced form of Gobiomys (Rodentia), and is most probably Sharamurunian in age. Thus, Wulanhuxiu documents replacement, albeit incomplete, of paleoplacentals by neoplacentals in the Chinese Eocene record

The mammalian skull: Development, structure and function

The mammalian skull is an informative and versatile study system critical to research efforts across the broad spectrum of molecular, cellular, organismal and evolutionary sciences. The amount of knowledge concerning mammalian skull continues to grow exponentially, fuelled by the advent of new research methods and new material. Computed microtomography, including X-ray imaging using synchrotron radiation, proved to be an important tool for the descriptive and quantitative analysis of cranial anatomy. A major conceptual change, namely combining genetics and development with evolution into ‘evo-devo’ studies, also contributed to our knowledge of the mammalian skull enormously. These advances, coupled with novel techniques now allow researchers to integrate the process of cranial development with data from the fossil record, which is also augmented by seminal discoveries from Africa, Asia and both Americas. However, for decades, there has been no comprehensive source covering fundamental aspects of this vibrant field of evolutionary biology. To address this gap, we offer in this theme issue a balanced mix of research papers and reviews from leading experts in the field and a younger generation of scientists from five continents