Variation of the Schmelzmuster and Other Enamel Characters in Molars of the Primitive Pliocene Vole Ogmodontomys from Kansas

Variation in enamel microhistology (the schmelzmuster) and other features were examined in adult Ogmodontomys poaphagus molars from the Meade Basin of southwestern Kansas. Measurements of total enamel thickness and thickness of tangential enamel were taken on individual triangles of first upper molar samples from the superposed Ripley B, Hornet and Rexroad 2A local faunas. Although thickness of total enamel layers and the tangential zone were not statistically different between triangles and sites, the quality of tangential enamel on the trailing edges increased, from incipient tangential and well-developed tangential enamel (Ripley B and Hornet) to solely well-developed tangential enamel (Rexroad 2A). The fundamental schmelzmuster of O. poaphagus is radial enamel on the leading edges and radial plus incipient or well-developed tangential enamel on the trailing edges. These results support the assertion that the schmelzmuster is a consistent and genetically controlled character mosaic, and confirm suggestions in the literature that Ogmodontomys is distinct from Mimomys. Paired sample Student’s t-tests of trailing/leading edge samples for each triangle were used to test for differentiation. Although the molars of Ogmodontomys poaphagus appear undifferentiated under the light microscope, and are indeed separable from the distinctly negatively differentiated molars of Mimomys by this technique, our results show that the M1s of O. poaphagus are slightly negatively differentiated, with various triangles being either undifferentiated or negatively differentiated. O. sawrockensis has a more primitive schmelzmuster than O. poaphagus, and the presence of discrete (primitive) lamellar enamel on the apices of at least one triangle in 55% of the sample from Fallen Angel B, combined with the absence of lamellar enamel in molars of O. poaphagus from Fox Canyon and all subsequent Ogmodontomys samples, conclusively demonstrates that the Fallen Angel B population of O. sawrockensis was not ancestral to O. poaphagus in the Meade Basin. A unique mosaic of dental and size features indicates that the early Blancan O. pipecreekensis from Indiana is a sister species to O. sawrockensis and not ancestral to O. poaphagus.     

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.     

Systematic Position of the African Dormouse Graphiurus (Rodentia, Gliridae) Assessed from Cytochrome b and 12S rRNA Mitochondrial Genes

Gliridae is a small family of rodents including three subfamilies: the Eurasian Glirinae (with three genera) and Leithiinae (with four genera) and the African Graphiurinae (with a single genus). Phylogenetic relationships among these eight genera are not fully resolved based on morphological characters. Moreover, the genus Graphiurus is characterized by numerous peculiar features (morphological characters and geographical distribution), raising the question of its relationships to the family Gliridae. The phylogenetic position of Graphiurus and the intra-Gliridae relationships are here addressed by a molecular analysis of 12S RNA and cytochrome b mitochondrial gene sequences for six glirid genera. Phylogenetic analyses are performed with three construction methods (neighbor-joining, maximum parsimony and maximum likelihood) and tests of alternative topologies with respect to the most likely. Our analyses reveal that Graphiurus is clearly a member of the Gliridae, refuting the hypothesis that the family could be paraphyletic. Among Gliridae, phylogenetic relationships are poorly resolved: the Leithiinae could be monophyletic, there is no support for the subfamily Glirinae, and the closest relative of Graphiurus is not identified. The inclusion of Graphiurus among Gliridae allows us to postulate that its hystricomorphous condition has been achieved convergently with other hystricomorphous rodents.     

Evolution of the zygomasseteric construction in Rodentia,as revealed by a geometric morphometric analysis of the mandible of Graphiurus (Rodentia,Gliridae)

Graphiurus is a peculiar taxon among the monophyletic Gliridae (order Rodentia) in showing hystricomorphy of the zygomasseteric architecture of the skull [large infraorbital foramen (IOF), and correlative muscular arrangements). We analysed 34 extant genera taken from two groups of sciurognath rodents that share a large IOF (hystricomorph and myomorph) using elliptical Fourier transform in order to appraise whether this feature of cranial morphology was also accompanied by similar changes in mandible shape. The mandible of Graphiurus is distinct from those of all other hystricomorph sciurognath rodents in showing a more elongated coronoid process and a shorter angular process. Thus, two distinct zygomasseteric organizations (i.e. myomorphy and hystricomorphy of graphiurines) are associated with a similar mandible shape characterized by a well‐developed coronoid process. Results show that hystricomorphy of graphiurines was achieved convergently with other hystricomorph rodents. Protrogomorphy is the plesiomorphic condition in Gliridae and hystricomorphy is an autapomorphic feature of Graphiurus. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154 , 807–821.     

Contributions to the knowledge of sieve-element plastids inGunneraceae and allied families

P-type sieve-element plastids were found in theGunneraceae, while S-type plastids are present in theHaloragaceae andHippuridaceae. The specific characters of the sieve-element plastids (e.g., their size and the morphology of their contents) are discussed in relation to other taxa of theRosidae containing P-type plastids and to the systematic position of theGunneraceae. Contributions to the Knowledge of P-Type Sieve-Element Plastids in Dicotyledons, III. — For other parts of this series see (I.:)Behnke (1982 b) and (II.:)Behnke (1985).     

Structure and composition of the incisor enamel of extant and fossil mammals with tooth pigmentation

The inclusion of iron compounds in teeth, which impart a red to orange colour to them, is a phenomenon shown by several groups of vertebrates in different periods of their evolution. Incisors from fossil and extant shrews and from extant rodents were sectioned and studied with the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to compare their structure and the distribution of Fe. The enamel in white‐ and red‐toothed soricids has three layers; two of them are divided into two zones in the red‐toothed species. However, the most external layer varies among taxa; it is well defined in Sorex but difficult to identify in the Early Pleistocene genera Beremendia or Dolinasorex. In the arvicoline rodent Terricola, only two layers can be defined, the outer of which is divided into two zones depending on the presence or absence of Fe. The Fe proportions in the larger soricids reach up to 45%, but in rodents only up to 10% (weight % with respect to Fe + Ca + P). The STEM study shows that in a fossil soricid the Fe phases form clusters of nanometric particles of very poor crystalline oxides or hydroxides surrounding the apatite crystals that form the enamel.     

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.     

Variation in enamel development of South African fossil hominids

Dental tissues provide important insights into aspects of hominid palaeobiology that are otherwise difficult to obtain from studies of the bony skeleton. Tooth enamel is formed by ameloblasts, which demonstrate daily secretory rhythms developing tissue-specific structures known as cross striations, and longer period markings called striae of Retzius. These enamel features were studied in the molars of two well known South African hominid species, Australopithecus africanus and Paranthropus robustus. Using newly developed portable confocal microscopy, we have obtained cross striation periodicities (number of cross striations between adjacent striae) for the largest sample of hominid teeth reported to date. These data indicate a mean periodicity of seven days in these small-bodied hominids. Important differences were observed in the inferred mechanisms of enamel development between these taxa. Ameloblasts maintain high rates of differentiation throughout cervical enamel development in P. robustus but not in A. africanus. In our sample, there were fewer lateral striae of Retzius in P. robustus than in A. africanus. In a molar of P. robustus, lateral enamel formed in a much shorter time than cuspal enamel, and the opposite was observed in two molars of A. africanus. In spite of the greater occlusal area and enamel thickness of the molars of both fossil species compared with modern humans, the total crown formation time of these three fossil molars was shorter than the corresponding tooth type in modern humans. Our results provide support for previous conclusions that molar crown formation time was short in Plio-Pleistocene hominids, and strongly suggest the presence of different mechanisms of amelogenesis, and thus tooth development, in these taxa.     

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.     

Functional symmetries in the schmelzmuster and morphology of rootless rodent molars

Morphology and schmelzmuster of rootless cheek teeth of 25 extant rodent genera were studied in relation to jaw movement. A differentiation between leading and trailing edges is observed regularly in enamel thickness and schmelzmuster. Similarities between antagonists are interpreted as 'functional symmetries'. Differences in the enamel thickness, the schmelzmuster and orientation of cutting edges are controlled by functional and phylogenetic constraints. The heterogenous sample allows discrimination between these two constraints. The most obvious functional constraint leads to the almost regular occurrence of radial enamel on the push sides of cutting edges. The degree of functional symmetry seems to be determined by phylogenetic limitations.     

Enamel thickness,microstructure and development in Afropithecus turkanensis

Afropithecus turkanensis, a 17-17.5 million year old large-bodied hominoid from Kenya, has previously been reported to be the oldest known thick-enamelled Miocene ape. Most investigations of enamel thickness in Miocene apes have been limited to opportunistic or destructive studies of small samples. Recently, more comprehensive studies of enamel thickness and microstructure in Proconsul, Lufengpithecus, and Dryopithecus, as well as extant apes and fossil humans, have provided information on rates and patterns of dental development, including crown formation time, and have begun to provide a comparative context for interpretation of the evolution of these characters throughout the past 20 million years of hominoid evolution. In this study, enamel thickness and aspects of the enamel microstructure in two A. turkanensis second molars were quantified and provide insight into rates of enamel apposition, numbers of cells actively secreting enamel, and the time required to form regions of the crown. The average value for relative enamel thickness in the two molars is 21.4, which is a lower value than a previous analysis of this species, but which is still relatively thick compared to extant apes. This value is similar to those of several Miocene hominoids, a fossil hominid, and modern humans. Certain aspects of the enamel microstructure are similar to Proconsul nyanzae, Dryopithecus laietanus, Lufengpithecus lufengensis, Graecopithecus freybergi and Pongo pygmaeus, while other features differ from extant and fossil hominoids. Crown formation times for the two teeth are 2.4-2.6 years and 2.9-3.1 years respectively. These times are similar to a number of extant and fossil hominoids, some of which appear to show additional developmental similarities, including thick enamel. Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enamelled extant apes.     

The evolution of dinosaur tooth enamel microstructure

The evolution of tooth enamel microstructure in both extinct and extant mammalian groups has been extensively documented, but is poorly known in reptiles, including dinosaurs. Previous intensive sampling of dinosaur tooth enamel microstructure revealed that: (1) the three‐dimensional arrangement of enamel types and features within a tooth—the schmelzmuster—is most useful in diagnosing dinosaur clades at or around the family level; (2) enamel microstructure complexity is correlated with tooth morphology complexity and not necessarily with phylogenetic position; and (3) there is a large amount of homoplasy within Theropoda but much less within Ornithischia. In this study, the examination of the enamel microstructure of 28 additional dinosaur taxa fills in taxonomic gaps of previous studies and reinforces the aforementioned conclusions. Additionally, these new specimens reveal that within clades such as Sauropodomorpha, Neotheropoda, and Euornithopoda, the more basal taxa have simpler enamel that is a precursor to the more complex enamel of more derived taxa and that schmelzmusters evolve in a stepwise fashion. In the particularly well‐sampled clade of Euornithopoda, correlations between the evolution of dental and enamel characters could be drawn. The ancestral schmelzmuster for Genasauria remains ambiguous due to the dearth of basal ornithischian teeth available for study. These new specimens provide new insights into the evolution of tooth enamel microstructure in dinosaurs, emphasizing the importance of thorough sampling within broadly inclusive clades, especially among their more basal members.     

New Pliocene remains of Camelus grattardi (Mammalia,Camelidae) from the Shungura Formation,Lower Omo Valley,Ethiopia, and the evolution of African camels

Abstract

Camels are exceptionally rare in the Plio-Pleistocene fossil record of Africa, hindering attempts to understand the evolution of this family on the continent. Here we describe recently collected camel specimens from the Shungura Formation, Lower Omo Valley, Ethiopia, and attribute these remains to Camelus grattardi. The new specimens date to the late Pliocene (~3 to 2.6 Ma) and consist of three upper molars, one upper premolar, and two proximal metatarsals. The dental specimens confirm this species’ small P4 relative to its molars, a trait that differs significantly from all extant and fossil Old World camels. The metatarsals indicate that C. grattardi was similar in size to the living Bactrian camel, C. bactrianus. Phylogenetically, we find no suitable ancestor, sister, or descendant of the eastern African fossil camel, which suggests greater lineage diversity in Plio-Pleistocene Camelus than previously recognised. Microwear analyses suggest that C. grattardi was likely a mixed-feeder preferring browse, which is consistent with carbon isotopes of enamel from the Turkana Basin. A review of the fossil record of African camels suggests no clear paleoenvironmental association, as fossil camels occur in a range of environments from dry savannas with no permanent water bodies to closed woodlands along the paleo-Omo River.     

Brief communication: enamel thickness trends in the dental arcade of humans and chimpanzees

In addition to evidence for bipedality in some fossil taxa, molar enamel thickness is among the few characters distinguishing (thick-enameled) hominins from the (thin-enameled) African apes. Despite the importance of enamel thickness in taxonomic discussions and a long history of scholarship, measurements of enamel thickness are performed almost exclusively on molars, with relatively few studies examining premolars and anterior teeth. This focus on molars has limited the scope of enamel thickness studies (i.e., there exist many fossil hominin incisors, canines, and premolars). Increasing the available sample of teeth from which to compare enamel thickness measurements from the fossil record could substantially increase our understanding of this aspect of dental biology, and perhaps facilitate greater taxonomic resolution of early hominin fossils. In this study, we report absolute and relative (size-scaled) enamel thickness measurements for the complete dentition of modern humans and chimpanzees. In accord with previous studies of molars, chimpanzees show lower relative enamel thickness at each tooth position, with little overlap between the two taxa. A significant trend of increasing enamel thickness from anterior to posterior teeth is apparent in both humans and chimpanzees, indicating that inter-taxon comparisons should be limited to the same tooth position in order to compare homologous structures. As nondestructive imaging techniques become commonplace (facilitating the examination of increasing numbers of fossil specimens), studies may maximize available samples by expanding beyond molars.     

Gliridae (Rodentia,Mammalia) with a simple dental pattern: a new genus and new species from the European Early and Middle Miocene

A new genus of Gliridae, Simplomys gen. nov. is proposed. It contains glirids with a simplified dental pattern from the European Early and Middle Miocene. Simplomys gen. nov. includes several species originally described as Pseudodryomys such as Simplomys simplicidens, Simplomys robustus, Simplomys julii, and Simplomys aljaphi. In addition, a new species, Simplomys meulenorum sp. nov. , is proposed from the Spanish Miocene. The species of this genus share not only a very reduced and simplified dental morphology, but also unique dental proportions that clearly separate them from any other genera of Gliridae. Simplomys gen. nov. is recorded in most of the fossil faunas from the Early and Middle Miocene of the Iberian Peninsula, and shows the maximum diversity in this area during Mammal Neogene Zones MN 3 and MN 4. The genus has been also recorded in other European countries such as France, Germany, and Switzerland, conferring to this very characteristic taxon an important role for biochronological correlations within the European continent. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 157 , 622–652.     

亚热带地区竞争型和忍耐型树种叶片可溶性有机质数量及光谱学特征

气候变化下,不同生态策略的树种对环境变化有着不同的响应能力,影响其叶片淋溶产生的DOM(Dissolved organic matter)的数量和质量,进而影响土壤的养分循环。通过探究亚热带地区不同生态策略树种叶片DOM数量及光谱学特征的差异,评估不同数量和结构特征DOM输入到土壤对养分循环的影响。本研究选取6种树种鲜叶进行浸提,其中竞争型(Competitive,C)和忍耐型(Stress-tolerant,S)各3种(树参(Dendropanax dentiger),黄绒润楠(Machilus grijsii),黄牛奶树(Symplocos cochinchinensis(Lour.)),细柄阿丁枫(Altingia gracilipes),丝栗栲(Castanopsis fargesii)和罗浮栲(Castanopsis faberi))。通过溶解性有机碳(Dissolved organic carbon,DOC)、溶解性有机氮(Dissolved organic nitrogen,DON)表征DOM的数量特征,通过紫外吸收值(Special Ultraviolet-Visible Absorption,SUVA),腐殖化指标(Humification index,HIX)和傅里叶红外光谱(Fouriertransform infrared,FTIR)等光谱指标表征DOM质量特征。结果表明:不同生态策略树种的叶浸提液中可溶性有机碳浓度无显著差异,但是C策略树种浸提液中可溶性有机氮浓度大于S策略的DON浓度。此外,S策略的芳香化指数(Aromatic index,AI)和腐殖化指数(HIX)均高于C策略。C策略树种的发射荧光强度也高于S策略,说明C策略树种DOM腐殖化程度较低,易分解物质含量高;S策略难分解物质多,腐殖化程度较高。傅里叶红外光谱结果表明,各树种叶浸提的DOM存在相似的吸收峰,其中以H键键合的—OH伸缩振动最强且C策略树种结果相对简单,验证了荧光光谱的结果。总体而言,与C策略相比,S策略树种叶片浸提的DOM结构更复杂,养分含量更高。这可能是因为,S策略树种对环境变化具有更高的适应性。由于其DOM结构相对复杂,输入土壤后减缓土壤碳周转速率,在未来气候变化情景下,S策略树种可能有利于土壤碳汇的形成。     

Dental wear,wear rate,and dental disease in the African apes

The African apes possess thinner enamel than do other hominoids, and a certain amount of dentin exposure may be advantageous in the processing of tough diets eaten by Gorilla. Dental wear (attrition plus abrasion) that erodes the enamel exposes the underlying dentin and creates additional cutting edges at the dentin‐enamel junction. Hypothetically, efficiency of food processing increases with junction formation until an optimal amount is reached, but excessive wear hinders efficient food processing and may lead to sickness, reduced fecundity, and death. Occlusal surfaces of molars and incisors in three populations each of Gorilla and Pan were videotaped and digitized. The quantity of incisal and molar occlusal dental wear and the lengths of dentin–enamel junctions were measured in 220 adult and 31 juvenile gorilla and chimpanzee skulls. Rates of dental wear were calculated in juveniles by scoring the degree of wear between adjacent molars M1 and M2. Differences were compared by principal (major) axis analysis. ANOVAs compared means of wear amounts. Pearson correlation coefficients were calculated to compare the relationship between molar wear and incidence of dental disease. Results indicate that quantities of wear are significantly greater in permanent incisors and molars and juvenile molars of gorillas compared to chimpanzees. The lengths of dentin–enamel junctions were predominantly suboptimal. Western lowland gorillas have the highest quantities of wear and the most molars with suboptimal wear. The highest rates of wear are seen in Pan paniscus and Pan t. troglodytes, and the lowest rates are found in P.t. schweinfurthii and G. g. graueri. Among gorillas, G. b. beringei have the highest rates but low amounts of wear. Coefficients between wear and dental disease were low, but significant when all teeth were combined. Gorilla teeth are durable, and wear does not lead to mechanical senescence in this sample. Am. J. Primatol. 72:481–491, 2010. © 2010 Wiley‐Liss, Inc.     

Rodent phylogeny and a timescale for the evolution of Glires: evidence from an extensive taxon sampling using three nuclear genes

Rodentia is the largest order of placental mammals, with approximately 2,050 species divided into 28 families. It is also one of the most controversial with respect to its monophyly, relationships between families, and divergence dates. Here, we have analyzed and compared the performance of three nuclear genes (von Willebrand Factor, interphotoreceptor retinoid-binding protein, and Alpha 2B adrenergic receptor) for a large taxonomic sampling, covering the whole rodent and placental diversity. The phylogenetic results significantly support rodent monophyly, the association of Rodentia with Lagomorpha (the Glires clade), and a Glires + Euarchonta (Primates, Dermoptera, and Scandentia) clade. The resolution of relationships among rodents is also greatly improved. The currently recognized families are divided here into seven well-defined clades (Anomaluromorpha, Castoridae, Ctenohystrica, Geomyoidea, Gliridae, Myodonta, and Sciuroidea) that can be grouped into three major clades: Ctenohystrica, Gliridae + Sciuroidea, and a mouse-related clade (Anomaluromorpha, Castoridae + Geomyoidea, and Myodonta). Molecular datings based on these three genes suggest that the rodent radiation took place at the transition between Paleocene and Eocene. The divergence between rodents and lagomorphs is placed just at the K-T boundary and the first splits among placentals in the Late Cretaceous. Our results thus tend to reconcile molecular and morphological-paleontological insights.     

Variation in enamel thickness within the genus Homo

Recent humans and their fossil relatives are classified as having thick molar enamel, one of very few dental traits that distinguish hominins from living African apes. However, little is known about enamel thickness in the earliest members of the genus Homo, and recent studies of later Homo report considerable intra- and inter-specific variation. In order to assess taxonomic, geographic, and temporal trends in enamel thickness, we applied micro-computed tomographic imaging to 150 fossil Homo teeth spanning two million years. Early Homo postcanine teeth from Africa and Asia show highly variable average and relative enamel thickness (AET and RET) values. Three molars from South Africa exceed Homo AET and RET ranges, resembling the hyper thick Paranthropus condition. Most later Homo groups (archaic European and north African Homo, and fossil and recent Homo sapiens) possess absolutely and relatively thick enamel across the entire dentition. In contrast, Neanderthals show relatively thin enamel in their incisors, canines, premolars, and molars, although incisor AET values are similar to H. sapiens. Comparisons of recent and fossil H. sapiens reveal that dental size reduction has led to a disproportionate decrease in coronal dentine compared with enamel (although both are reduced), leading to relatively thicker enamel in recent humans. General characterizations of hominins as having ‘thick enamel’ thus oversimplify a surprisingly variable craniodental trait with limited taxonomic utility within a genus. Moreover, estimates of dental attrition rates employed in paleodemographic reconstruction may be biased when this variation is not considered. Additional research is necessary to reconstruct hominin dietary ecology since thick enamel is not a prerequisite for hard-object feeding, and it is present in most later Homo species despite advances in technology and food processing.     

Unique differentiation of radial enamel in Arsinoitherium (Embrithopoda,Tethytheria)

The enamel microstructure in molars of Arsinoitherium is reinvestigated and a new modification of radial enamel (RE), ‘arsinoitheriid radial enamel (ARE)’, is defined. It is characterised by alternating stripes with different organisation of the interprismatic matrix but no prism decussation. Recognition of this new subtype leads to a reinterpretation of structure previously identified as modified radial enamel and of Hunter–Schreger bands in Arsinoitherium. The newly differentiated ARE of Arsinoitherium is more derived in relation to corresponding microstructures of Palaeoamasia and Crivadiatherium. A careful reinvestigation of RE in other Paenungulata will be required to provide additional data bearing on phylogenetic reconstruction. The enamel of Phenacolophus argues against inclusion of this genus in the Embrithopoda.