Incisor Schmelzmuster Diversity in South America's Oldest Rodent Fauna and Early Caviomorph History

Investigation of the enamel microstructure of 20 isolated rodent incisors from the ?Eocene Santa Rosa local fauna (Peru) yielded exclusively schmelzmuster with multiserial Hunter–Schreger bands (HSB). All three subtypes of multiserialHSB with parallel, acute angular, and rectangular interprismatic matrix (IPM) that were previously reported for caviomorph rodents are present. Two lower incisors with rectangular IPM can be attributed to the Octodontoidea, a caviomorph superfamily exhibiting this highly derived enamel type. The plesiomorphic pauciserial condition that characterizes early Paleogene rodents such as North American Ischyromyoidea (including “Franimorpha”) has not been detected. It is therefore probable that the founder populations of South American Caviomorpha already possessed a derived incisor schmelzmuster with multiserial HSB that is shared with African Thryonomyoidea. Because on the North American continent a possible stem-lineage representative of Caviomorpha with multiserial HSB has never been detected, incisor enamel microstructure supports the hypothesis of an African origin of Caviomorpha from a common ancestor shared with Thryonomyoidea.     

The enamel microstructure ofAnchitheriomys (Rodentia, Mammalia) in comparison with that of other beavers and of porcupines

In the enamel microstructure of the incisors, porcupines (Hystricidae) are characterized by multiserial Hunter-Schreger bands (HSB) while beavers (Castoridae) basically have uniserial HSB. In addition, two groups can be differentiated among beavers based on enamel microstructure.Palaeocastor, Steneofiber andCastor show rather typical uniserial HSB. OnlyCastor shows a slight tendency of fusion of the bands. In contrast,Trogontherium andCastoroides are characterized by the regular occurrence of fused bands.Anchitheriomys, having uniserial HSB and a high frequency of fused bands fits well with theTrogontherium group, proving the castorid nature ofAnchitheriomys. Furthermore, the enamel differentiation observed among the Castoridae thus provides an important character for further systematic subdivision of the beavers. In the enamel of the cheek teeth, porcupines can be told apart from beavers as well.Anchitheriomys shows a two-layered schmelzmuster with inner radial enamel like other beavers. Porcupines have thick HSB throughout the enamel layer. This character in the cheek teeth facilitates the separation of isolated beaver teeth from porcupine teeth.     

Structural patterns of enamel in the superfamily Ceboidea

A fundamental enamel structure was found in the superfamily Ceboidea, and tentatively named the nonserial pattern, as distinguished from the multiserial and uniserial patterns. In the nonserial pattern, almost all rows of enamel prisms are straight to slightly curved from the enameldentin junction to the surface of the tooth, and are nearly uniformly oriented. Accordingly, Schreger's band is definitely lacking throughout the length and width of enamel. The interprismatic bundles between both adjacent rows are well developed. The nonserial pattern has so far been found to be limited to some genera of which the molars preserve rather primitive external features; i.e.,Saimiri, Callicebus, Aotus, Alouatta, Saguinus andLeontopithecus. On the other hand, the multiserial pattern is fairly common in many genera of the Ceboidea. The nonserial pattern, which corresponds to the pattern I described byBoyde (1964), is the most primitive, judging from its occurrence, structural simplicity and other factors. The multiserial and uniserial patterns are assumed to be independently evolved from the nonserial pattern.     

Enamel microstructure ofTribosphenomys (Mammalia,glires): Character analysis and systematic implications

Enamel distribution on the upper and lower incisors ofTribosphenomys minutus (from Late Paleocene-Early Eocene of Inner Mongolia of China) is typically rodent-like, i.e., primarily confined to the anterior surface throughout these transversely compressed, evergrowing teeth. AlthoughTribosphenomys incisor enamel is differentiated into two layers, it does not possess Hunter-Schreger bands (HSB). The incisor and molar enamels are radial in type, a condition regarded as either an autapomorph or a primtive retention forTribosphenomys. Character polarities concerning enamel thickness, enamel layer number, HSB, enamel types, and functional and phylogenetic implications of the enamel structures are discussed. Overall, enamel microstructural evolution at high taxonomic levels within Glires displays considerably more homoplasy than generally appreciated. A phylogenetic definition of Rodentia is proposed.Tribosphenomys is the sister-group of a taxon here named Rodentia, and thus is not itself a member of the order, from a systematic viewpoint.     

Evolution of the tooth enamel microstructure in the earliest proboscideans (Mammalia)

Microstructural features of the mammalian tooth enamel are rarely used to construct phylogenies, although macromorphological characters of the dentition figure prominently in phylogenetic analysis. In order to test the phylogenetic significance of the enamel microstructures, we investigate here the earliest proboscideans recently found in the Early Palaeogene of Africa (e.g. Phosphatherium , Daouitherium , Khamsaconus , and Numidotherium ). The results are discussed in the light of the recent advances concerning the intra- and interordinal relationships of the Proboscidea. We also consider other basal paenungulates such as 'anthracobunids', embrithopods, and hyraxes. The analysed microstructures suggest that the enamel ancestral morphotype of paenungulates was primitive for eutherian mammals, consisting in radial enamel. Some basal proboscideans developed decussations of prisms in Hunter-Schreger bands (HSB), as did most of the medium to large-sized mammals. More evolved proboscideans developed very complex enamel, the 3-D enamel, which represents an apomorphy for the group. The three-layered Schmelzmuster, typical of the elephantoids (3-D enamel, HSB, and radial enamel), is acquired during the late Eocene with the enigmatic ' Numidotherium ' savagei . This species is here considered as an advanced proboscidean along with Moeritherium -Deinotheriidae-Elephantiformes. The peculiar enamel of elephantoids arose step by step. Although homoplasy and mosaic evolution occur, the enamel microstructures represent an important source of new dental characters for phylogenetic reconstructions. As macromorphological characters testified, the diversity of the enamel microstructures observed in the various basal proboscideans illustrates an unexpected early diversity of the order in Africa.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 611–628.     

Variance of molecular datings, evolution of rodents and the phylogenetic affinities between Ctenodactylidae and Hystricognathi

The von Willebrand factor (vWF) gene has been used to understand the origin and timing of Rodentia evolution in the context of placental phylogeny vWF exon 28 sequences of 15 rodent families and eight non-rodent eutherian clades are analysed with two different molecular dating methods (uniform clock on a linearized tree; quartet dating). Three main conclusions are drawn from the study of this nuclear exon. First, Ctenodactylidae (gundis) and Hystricognathi (e.g. porcupines, guinea-pigs, chinchillas) robustly cluster together in a newly recognized clade, named 'Ctenohystrica'. The Sciurognathi monophyly is subsequently rejected. Pedetidae (springhares) is an independent and early diverging rodent lineage, suggesting a convergent evolution of the multiserial enamel of rodent incisors. Second, molecular date estimates are here more influenced by accuracy and choice of the palaeontological temporal references used to calibrate the molecular clock than by either characters analysed (nucleotides versus amino acids) or species sampling. The caviomorph radiation at 31 million years (Myr) and the pig porpoise split at 63 Myr appear to be reciprocally compatible dates. Third, during the radiation of Rodentia, at least three lineages (Gliridae, Sciuroidea and Ctenohystrica) emerged close to the Cretaceous-Tertiary boundary, and their common ancestor separated from other placental orders in the Late Cretaceous.     

Tooth Enamel Microstructure of Living and Extinct Hyracoids Reveals Unique Enamel Types Among Mammals

Among medium- to large-sized terrestrial ‘ungulates,’ there is often a relationship between increasing body size, correlated changes in diet, and increased complexity of the enamel microstructures [notably the development of Hunter-Schreger bands (HSB)]. An exhaustive survey of the enamel microstructures of living and extinct Hyracoidea demonstrates, however, that the Schmelzmuster within this order of mammals is generally one-layered and formed by radial enamel despite a large range of body sizes and dietary adaptations; HSB are remarkably absent. Radial enamel is characteristic of early diverging hyracoids, as well as more derived members of the extinct families Geniohyidae and Pliohyracidae, and the extant Procaviidae. Only some large ‘Saghatheriidae,’ and all members of the family Titanohyracidae, developed a more complex enamel microstructure (i.e., with prisms decussating), a unique condition among Mammalia that we name ‘bundled enamel’ (BE). This structure is reminiscent to some degree of both the ‘Pyrotherium enamel’ and the ‘3D enamel’ of proboscideans. Hyracoids with BE represented a major component of the diversity of mid- to large-sized herbivores during the Paleogene in Africa. Like HSB, which are developed by most other ‘ungulates,’ the BE is regarded as a device for resisting propagation of cracks during mastication. Hyracoids never developed however the ‘modified radial enamel’ that is characteristic of most large and hypsodont perissodactyls and artiodactyls that entered Africa during the Miocene.     

Colony Growth Pattern in Electra pilosa (Linnaeus) and Comparable Encrusting Cheilostome Bryozoans

Colony growth pattern is described in E. pilosa, an abundant cheilostome bryozoan commonly found as an epiphyte of Laminaria. Each zooid has 4 potential budding loci—one distal, two lateral and one proximal. The ancestrula buds daughter zooids from all of these loci; the two lateral buds appear first, followed by the distal bud and, after a long delay, the proximal bud. The laterally budded zooids curve inwards as they grow to form a triad with their distally budded sibling zooid. ‘Mature’ multiserial colonies growing on flat substrata consist of a series of radially diverging sectors. Each sector has an axis, generally of 3 parallel rows of zooids, flanked by wings consisting of rows of zooids originating as lateral buds from the section axis which infills the area between the axes. Occasional colonies occur with uniserial or semiuniserial growth patterns. These resemble colonies of the obligatory uniserial species Pyripora catenularia and poorly fed colonies of the related Conopeum tenuissimum, which is normally multiserial like E. pilosa. The ‘composite multiserial’ colonies of E. pilosa differ in several respects from ‘unitary multiserial’ colonies characteristic of most sheet-like cheilostomes, including the well-known Membranipora membranacea. Composite and unitary multiserial growth patterns may have evolved independently from uniserial ancestors.     

亚洲始新世啮齿目一新科——争胜鼠科(Zelomyidae)(英文)

继 1 990年王伴月和李春田记述了发现在吉林桦甸中始新世的争胜鼠 (Zelomys)之后 ,近年又在山西垣曲、河南卢氏、江苏溧阳等地的中始新世到晚始新世 (Irdinmanhan lateShara murunianorErgilian)的 5个地点中陆续发现了一些与争胜鼠相近的新材料。经研究后 ,将所有材料归诸于创建的一新科 :争胜鼠科 (Zelomyidaefam .nov .)。它包括了争胜鼠及本文新记述的安氏鼠 (Andersomysgen.nov.)、耗子 (Haozigen .nov .)和苏鼠 (Suomysgen .nov.)共 4属 6种。新科的特征是 :始啮型头骨 -松鼠型下颌 ;门齿釉质层散系 ;颊齿具有发育的次尖和下次脊 ,后期种类上颊齿的外侧齿尖的唇侧发展成平凹到新月形。新科的系统关系不很清楚 ,尽管与始鼠科 (Eomyidae)有某些相似之处 ,但它更可能是独立发展的一个支系。争胜鼠属化石分布较广 ,在我国吉林、江苏和山西的中、晚始新世地层中都有发现 ,它的特征是下颊齿p4 m2宽度逐渐增大 ,p4 m3具下前边尖 ,并与短的下原尖前臂相接。属中除属型种———东方争胜鼠 (Z .orientalisWangetLi)外 ,还建立了一个约翰争胜鼠新种 (Z .joannessp .nov.) ,一约翰争胜鼠相似种 (Z .cf.Z .joannes)。同时认为Z .gracilisWangetLi,1 990是Z .orientalis的同物异名。约翰种与属型种的区别在于个?     

A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate,Mammalia) - Implications for Multituberculate Biology and Phylogeny

Tooth enamel microstructure is a reliable and widely used indicator of dietary interpretations and data for phylogenetic reconstruction, if all levels of variability are investigated. It is usually difficult to have a thorough examination at all levels of enamel structures for any mammals, especially for the early mammals, which are commonly represented by sparse specimens. Because of the random preservation of specimens, enamel microstructures from different teeth in various species are often compared. There are few examples that convincingly show intraspecific variation of tooth enamel microstructure in full dentition of a species, including multituberculates. Here we present a systematic survey of tooth enamel microstructures of Lambdopsalis bulla, a taeniolabidoid multituberculate from the Late Paleocene Nomogen Formation, Inner Mongolia. We examined enamel structures at all hierarchical levels. The samples are treated differently in section orientations and acid preparation and examined using different imaging methods. The results show that, except for preparation artifacts, the crystallites, enamel types, Schmelzmuster and dentition types of Lambdopsalis are relatively consistent in all permanent teeth, but the prism type, including the prism shape, size and density, may vary in different portions of a single tooth or among different teeth of an individual animal. The most common Schmelzmuster of the permanent teeth in Lambdopsalis is a combination of radial enamel in the inner and middle layers, aprismatic enamel in the outer layer, and irregular decussations in tooth crown area with great curvature. The prism seam is another comparably stable characteristic that may be a useful feature for multituberculate taxonomy. The systematic documentation of enamel structures in Lambdopsalis may be generalized for the enamel microstructure study, and thus for taxonomy and phylogenetic reconstruction, of multituberculates and even informative for the enamel study of other early mammals.     

Enamel Ultrastructure in Fossil Cetaceans (Cetacea: Archaeoceti and Odontoceti)

The transition from terrestrial ancestry to a fully pelagic life profoundly altered the body systems of cetaceans, with extreme morphological changes in the skull and feeding apparatus. The Oligocene Epoch was a crucial time in the evolution of cetaceans when the ancestors of modern whales and dolphins (Neoceti) underwent major diversification, but details of dental structure and evolution are poorly known for the archaeocete-neocete transition. We report the morphology of teeth and ultrastructure of enamel in archaeocetes, and fossil platanistoids and delphinoids, ranging from late Oligocene (Waitaki Valley, New Zealand) to Pliocene (Caldera, Chile). Teeth were embedded in epoxy resin, sectioned in cross and longitudinal planes, polished, etched, and coated with gold palladium for scanning electron microscopy (SEM) observation. SEM images showed that in archaeocetes, squalodontids and Prosqualodon (taxa with heterodont and nonpolydont/limited polydont teeth), the inner enamel was organized in Hunter-Schreger bands (HSB) with an outer layer of radial enamel. This is a common pattern in most large-bodied mammals and it is regarded as a biomechanical adaptation related to food processing and crack resistance. Fossil Otekaikea sp. and delphinoids, which were polydont and homodont, showed a simpler structure, with inner radial and outer prismless enamel. Radial enamel is regarded as more wear-resistant and has been retained in several mammalian taxa in which opposing tooth surfaces slide over each other. These observations suggest that the transition from a heterodont and nonpolydont/limited polydont dentition in archaeocetes and early odontocetes, to homodont and polydont teeth in crownward odontocetes, was also linked to a marked simplification in the enamel Schmelzmuster. These patterns probably reflect functional shifts in food processing from shear-and-mastication in archaeocetes and early odontocetes, to pierce-and-grasp occlusion in crownward odontocetes, with the implication of less demanding feeding biomechanics as seen in most extant odontocetes.     

Enamel microstructure defines a major Paleogene hippopotamoid clade: the Merycopotamini (Cetartiodactyla,Hippopotamoidea)

We present new material of the selenodont anthracothere Hemimeryx blanfordi from the Oligocene deposits of the Bugti Hills (Balochistan, Pakistan), collected between 1999 and 2002. This is the first undisputed Oligocene occurrence of the species, previously known from the early Miocene of Pakistan. Investigation of the molar enamel microstructure reveals a surprising mono-zonal Schmelzmuster, already detected in some middle to Late miocene selenedont anthracotheres. We include this observation combined with a morphological revision of H. blanfordi and a cladistic assessment of the dental evidence, to propose a new phylogenetic hypothesis regarding Hemimeryx and its close relatives. We confirm the clade including advanced bothriodontines, which we erect to a tribe rank and name Merycopotamini. The South Asian origin of Merycopotamini is consistent with hypothesized subsequent dispersal events of Merycopotamini from Asia to Africa.     

What isIvanantonia efremovi (Rodentia, Mammalia)?

Ivanantonia efremovi Shevyreva 1989, known from the Tsagan Khushu locality (Early Eocene of Mongolia), erroneously has been described asOrogomys obscurus Dashzeveg 1990. This form shows an interesting association of primitive and derived characters: the enamel of the incisors is two-layered and has pauciserial Hunter-Schreger bands in portio interna; the lower tooth row has only three jugal teeth; the tooth pattern of lower molars is primitive and looks very similar to that ofAlagomys, but the trigonid is less reduced with regards to the talonid. It also differs from other primitive rodents in the distribution of wear facets, suggesting that mandibular propalineal movement was pre-eminent during chewing. Comparisons ofIvanantonia with several Eocene-Oligocene rodents indicate possible relationships to the Early Oligocene North American genusNonomys.     

New records of archaic ungulates from the Lower Eocene of Sanshui Basin,Guangdong, China

Phenacolophidae is a group of little known archaic ungulates from the Late Paleocene to Middle Eocene of Asia. Its phylogenetic relationships with other altungulates have remained uncertain, partly because most phenacolophids are represented by poorly preserved material. Here we report a new phenacolophid, Sanshuilophus zhaoi gen. et sp. nov., from the Lower Eocene Huayong Formation of the Sanshui Basin, Guangdong, China. Although still fragmentary, the new specimens show that the new taxon is characterised by relatively large body size (except for Zaisanolophus), sub-molariform premolars, relatively higher bilophodont molars that lack the mesostyle, and tooth enamel microstructure with true prisms and typical Hunter-Schreger bands (HSB). With the new specimens and a review of the published phenacolophid material, we are able to provide an alternative identification for the tooth loci for the type specimen of Phenacolophus and further present an emended diagnosis for Phenacolophidae. The tooth morphology and enamel microstructure provide new evidence to support the notion that phenacolophids differ from species of Embrithopoda in having low-crown teeth, considerably slanting lophids, distinct paralophids and lacking the arsinoitheriid radial enamel. Phenacolophidae should not be included in Tethytheria but probably represent a stem group for altungulates, if not for all archaic ungulates.     

Enamel thickness and microstructure in pitheciin primates, with comments on dietary adaptations of the middle Miocene hominoid Kenyapithecus

Many living primates that feed on hard food have been observed to have thick-enameled molars. Among platyrrhine primates, members of the tribe Pitheciini (Cacajao, Chiropotes, and Pithecia) are the most specialized seed and nut predators, and Cebus apella also includes exceptionally hard foods in its diet. To examine the hypothesized relationship between thick enamel and hard-object feeding, we sectioned small samples of molars from the platyrrhine primates Aotus trivergatus, Ateles paniscus, Callicebus moloch, Cebus apella, Cacajao calvus, Chiropotes satanas, Pithecia monachus, and Pithecia pithecia. We measured relative enamel thickness and examined enamel microstructure, paying special attention to the development of prism decussation and its optical manifestation, Hunter-Schreger Bands (HSB). Cebus apella has thick enamel with well-defined but sinuous HSB overlain by a substantial layer of radial prisms. Aotus and Callicebus have thin enamel consisting primarily of radial enamel with no HSB, Ateles has thin enamel with moderately developed HSB and an outer layer of radial prisms, and the thin enamel of the pitheciins (Cacajao, Chiropotes, and Pithecia) has extremely well-defined HSB. Among platyrrhines, two groups that feed on hard objects process these hard foods in different ways. Cebus apella masticates hard and brittle seeds with its thick-enameled cheek teeth. Pitheciin sclerocarpic foragers open hard husks with their canines but chew relatively soft and pliable seeds with their molars. These results reveal that thick enamel per se is not a prerequisite for hard object feeding. The Miocene hominoid Kenyapithecus may have included hard objects in its diet, but its thick-enameled molars indicate that its feeding adaptations differed from those of the pitheciins. The morphology of both the anterior and posterior dentition, including enamel thickness and microstructure, should be taken into consideration when inferring the dietary regime of fossil species.     

Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea)

Longitudinal and cross sections of teeth from 17 species of the Recent dolphins (Delphinoidea and Inioidea) were examined under scanning electron microscope to study the arrangement and ultrastructure of dental tissues with reference to phylogenetic and functional constraints. For most species, enamel had a simple bi-layered structure of radial enamel and an outer layer of prismless enamel. The outer prismless layer varied from 5 to 30 % of enamel thickness. The enamel of Burmeister’s porpoise (Phocoena spinipinnis) was entirely prismless. The prisms had an open sheath; tubules and tuft-like structures were common at the enamel-dentine junction. Cetacean dentine was characterized by irregularly distributed dentinal tubules in a relatively homogenous dentinal matrix. Radial enamel was observed in all Delphinoidea and in the franciscana (Pontoporia blainvillei), whereas the Amazon river dolphin (Inia geoffrensis) had prisms organized in Hunter–Schreger bands. HSB in enamel are regarded as a device for resisting propagation of cracks. These may occur due to increased functional demands, possibly related to the hardness of the species diet. Simplification in tooth shape and reduced biomechanical demands plausibly explain the primitive radial organization among delphinoids and Pontoporia. The HSB structure in the Amazon river dolphin, similar to those of extinct archaeocetes, seems to have secondary functional implications. However, the distribution of HSB in more-basal odontocetes is too poorly known to judge whether the HSB of Inia are a retained plesiomorphic feature or convergence.     

The Schmelzmuster of the Paleogene South American rodentlike marsupialsGroeberia andPatagonia compared to rodents and other Marsupialia

The enamel of the lower incisors of two Paleogene South American rodent-like marsupialsGroeberia minoprioi (Groeberiidae) andPatagonia peregrina (Patagoniidae) is described. The fact that the Schmelzmuster lacks Hunter-Schreger bands proves that these genera are basically different from any rodent despite other morphological similarities. Although the enamel is like that of the kangaroo,Macropus rufus, it is not sufficient evidence to include these genera in the Marsupialia. Convergence, based on functional requirements, such as crack stopping mechanisms, is common in the evolution of enamel. Therefore, the Schmelzmuster testifies to the distinctness of the Groeberiidae and Patagoniidae from any rodent; their marsupial nature is based on craniomandibular characters.     

The enamel of Creodonta, Arctocyonidae, and Mesonychidae (Mammalia), with special reference to the appearance of Hunter-Schreger-Bands

The enamel structure of 16 creodont genera was examined by light microscopy and for 3 by scanning electron microscopy. All creodonts have prismatic enamel where prisms decussate in layers forming Hunter-Schreger-Bands (HSB). As only limited material was accessible to sectioning, the main emphasis is given to the appearance of HSB. As in other carnivorous mammals three types of HSB can be distinguished: undulating HSB, acute-angled HSB and zigzag HSB, which differ in their waviness in the horizontal course. Within the evolution of creodonts the amount of zigzag HSB in the teeth increased in hyaenodontids and oxyaenids respectively. This can be attributed to different loading conditions of the teeth due to various food types and the resulting bite forces. Tendencies to ossiphagous feeding habits correlate with zigzag HSB. The same general trends can be seen in 5 examined genera of the Arctocyonidae and Mesonychidae.     

New material of Diacodexis (Mammalia,Artiodactyla) from the early Eocene of Southern Europe

Diacodexeidae are the first representatives of Artiodactyla in the fossil record. Their first occurrence is at the very base of the Ypresian (earliest Eocene, 56.0 Ma) with Diacodexis, a genus well diversified during the early Eocene in Europe, especially during the MP7–MP8 + 9 interval. However, most of European species are documented by scarce material, retrieved from single localities. In this work, we describe new Diacodexis material from ～MP7 and ～MP8 + 9 localities of Southern Europe, including material of D. antunesi from Silveirinha, considered as the most primitive European Diacodexis species, and material from three localities from Southern France (Fordones, Palette, and La Borie). The new material documents Diacodexis premolar morphology and deciduous dentition which bear potentially important phylogenetic information, as well as astragali, including a specimen from Silveirinha that constitutes the earliest occurrence of an astragalus of the genus Diacodexis in the European fossil record. Investigation of the enamel microstructure reveals that early European species had a simple enamel pattern with one-layered Schmelzmuster composed of ‘basic’ radial enamel only, instead of the two-layered Schmelzmuster (thin radial enamel + thick layer of Hunter-Schreger bands) observed on North American species and so far considered to represent the primitive condition within Artiodactyla. In accordance with previous studies, our observations highlight that Diacodexis gigasei from Belgium is morphologically closer to the North American species D. ilicis than to D. antunesi from Portugal. The latter species, together with D. aff. antunesi from Fordones, appears to be morphologically closer to the Asiatic taxa D. indicus and D. pakistanensis. Finally, we found numerous similarities between D. cf. gigasei from Palette and D. gigasei, a result that challenges the intra-European provincialism that characterizes the earliest Ypresian. Diacodexis gigasei could be one of the rare species shared by the northwestern and southwestern European bioprovinces.     

Biomechanical adaptations for burrowing in the incisor enamel microstructure of Geomyidae and Heteromyidae (Rodentia: Geomyoidea)

The enamel microstructure of fossil and extant Geomyoidea (Geomyidae, Heteromyidae) lower incisors incorporates three‐ or two‐layered schmelzmusters with uniserial, transverse Hunter‐Schreger bands having parallel and perpendicular or exclusively perpendicular oriented interprismatic matrix. Phylogenetically, these schmelzmusters are regarded as moderately (enamel type 2) to highly derived (enamel type 3). Our analysis detected a zone of modified radial enamel close to the enamel–dentine junction. Modified radial enamel shows a strong phylogenetic signal within the clade Geomorpha as it is restricted to fossil and extant Geomyoidea and absent in Heliscomyidae, Florentiamyidae, and Eomyidae. This character dates back to at least the early Oligocene (early Arikareean, 29 Ma), where it occurs in entoptychine gophers. We contend that this specialized incisor enamel architecture developed as a biomechanical adaptation to regular burrowing activities including chisel‐tooth digging and a fiber‐rich diet and was probably present in the common ancestor of the clade. We regard the occurrence of modified radial enamel in lower incisors of scratch‐digging Geomyidae and Heteromyidae as the retention of a plesiomorphic character that is selectively neutral. The shared occurrence of modified radial enamel is a strong, genetically anchored argument for the close phylogenetic relationship of Geomyidae and Heteromyidae on the dental microstructure level.