Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens

Neanderthals have a distinctive suite of dental features, including large anterior crown and root dimensions and molars with enlarged pulp cavities. Yet, there is little known about variation in molar root morphology in Neanderthals and other recent and fossil members of Homo. Here, we provide the first comprehensive metric analysis of permanent mandibular molar root morphology in Middle and Late Pleistocene Homo neanderthalensis, and Late Pleistocene (Aterian) and recent Homo sapiens. We specifically address the question of whether root form can be used to distinguish between these groups and assess whether any variation in root form can be related to differences in tooth function. We apply a microtomographic imaging approach to visualise and quantify the external and internal dental morphologies of both isolated molars and molars embedded in the mandible (n = 127). Univariate and multivariate analyses reveal both similarities (root length and pulp volume) and differences (occurrence of pyramidal roots and dental tissue volume proportion) in molar root morphology among penecontemporaneous Neanderthals and Aterian H. sapiens. In contrast, the molars of recent H. sapiens are markedly smaller than both Pleistocene H. sapiens and Neanderthals, but share with the former the dentine volume reduction and a smaller root-to-crown volume compared with Neanderthals. Furthermore, we found the first molar to have the largest average root surface area in recent H. sapiens and Neanderthals, although in the latter the difference between M₁ and M₂ is small. In contrast, Aterian H. sapiens root surface areas peak at M₂. Since root surface area is linked to masticatory function, this suggests a distinct occlusal loading regime in Neanderthals compared with both recent and Pleistocene H. sapiens.     

Trigonid crests expression in Atapuerca-Sima de los Huesos lower molars: Internal and external morphological expression and evolutionary inferences

Trigonid crest patterning in lower molars is distinctive among Late Pleistocene hominins such as Homo neanderthalensis, fossil Homo sapiens and modern humans. In this paper, we present an examination of trigonid crest patterning in the Middle Pleistocene permanent lower molar sample (n = 62) of Homo heidelbergensis from Sima de los Huesos (SH). Crest expression was assessed from 3D models of the enamel and the dentine surfaces that were produced using micro-computed tomography (microCT). The aims of our analysis are to: 1) characterize the pattern of trigonid crest expression at the outer enamel and enamel-dentine junction surfaces (OES and EDJ) of the SH sample, 2) evaluate the concordance of expression between both surfaces, and 3) place trigonid crest variation in the SH sample into a phylogenetic context. Our results reveal a greater variability in the expression of trigonid crests at the EDJ (14 types) compared to the OES (4 types). Despite this variability, in almost all cases the expression of a continuous mid-trigonid or distal crest at the OES corresponds with the expression of a continuous mesial/mid-trigonid or distal trigonid crest, respectively, at the EDJ. Thus, it is possible to predict the type of trigonid crest pattern that would be at the OES in the case of partially worn teeth. Our study points to increased variability in trigonid crest expression in M₃s compared to M₁s and M₂s. Moreover, our analysis reveals that the SH sample matches broadly the trigonid crest patterns displayed by H. neanderthalensis and differs from those exhibited by H. sapiens, particularly in the almost constant expression of a continuous middle trigonid crest at the EDJ. However, SH hominins also exhibit patterns that have not been reported in H. neanderthalensis and H. sapiens samples. Other aspects of the variability of the trigonid crest expression at the dentine are presented and discussed.     

Application of synchrotron-radiation-based computer microtomography (SRμCT) to selected biominerals: embryonic snails, statoliths of medusae, and human teeth

Synchrotron-radiation-based computer microtomography (SRμCT) was applied to three biomineralised objects First, embryonic snails of the freshwater snail Biomphalaria glabrata, second, rhopalia (complex sense organs) of the medusa Aurelia aurita, and third, human teeth. The high absorption contrast between the soft tissue and mineralised tissues, i.e. the shell in the first case (consisting of calcium carbonate) and the statoliths in the second case (consisting of calcium sulphate hemihydrate), makes this method ideal for the study of biomineralised tissues. The objects can be non-destructively studied on a micrometre scale, and quantitative parameters like the thickness of a forming a snail shell or statolith crystal sizes can be obtained on a length scale of 1–2 μm. Using SRμCT, the dentin–enamel border can be clearly identified in X-ray dense teeth.     

Three-dimensional microarchitecture of the plates (primary, secondary, and carinar process) in the developing tooth of Lytechinus variegatus revealed by synchrotron X-ray absorption microtomography (microCT)

This paper reports the first noninvasive, volumetric study of entire cross-sections of a sea urchin tooth in which the individual calcite structural elements could be resolved. Two cross-sectionally intact fragments of a Lytechinus variegatus tooth were studied with synchrotron microCT (microcomputed tomography) with 1.66 microm voxels (volume elements). These fragments were from the plumula, that is the tooth zone with rapidly increasing levels of mineral; one fragment was from a position aboral of where the keel developed and the second was from the zone where the keel was developing. The primary plates, secondary plates, carinar process plates, prisms, and elements of the lamellar-needle complex were resolved. Comparison of the microCT data with optical micrographs of stained thin sections confirmed the identifications and measured dimensions of the characteristic microarchitectural features. The interplay of reinforcing structures (plates and prisms) was more clearly revealed in the volumetric numerical data sets than in single or sequential slices. While it is well known that the primary plates and prisms in camarodont teeth are situated to improve resistance to bending (which can be termed primary bending), the data presented provide a new understanding of the mechanical role of the carinar process plates, that is, a geometry consistent with that required in the keel to resist lateral or transverse bending of the tooth about a second axis. The increase in robustness of teeth incorporating lateral keel reinforcement suggests that the relative development of carinar processes (toward a geometry similar to that of L. variegatus) is a character which can be used to infer which sea urchins among the stirodonts are most primitive and among the camarodonts which are more primitive.     

Tubular microfossils from the Ediacaran Weng’an Biota (Doushantuo Formation,South China) are not early animals

The early Ediacaran Weng’an Biota (ca. 609 Ma) of the Doushantuo Formation (Guizhou Province, China) encompasses an abundant and exquisitely preserved assemblage of phosphatic microfossils that have provided unique insight into the origin and early evolution of multicellular eukaryotes. However, the affinities of these early organisms are far from certain, including the tubular microfossils Crassitubulus, Quadratitubus, Ramitubulus, and Sinocyclocylcicus. These taxa have been widely accepted as stem-cnidarians or, alternatively, interpreted as filamentous cyanobacteria, or multicellular algae. We use high-resolution X-ray tomographic microscopy to analyse the structure and development of the four taxa. Our data and analysis allow us to conclude that these four taxa were not biomineralized. Crassitubulus, Quadratitubus, and Sinocyclocylcicus, may be grouped on the basis that they exhibit alternating complete and incomplete cross walls, and bipolar growth; which makes them favourably comparable to filamentous cyanobacteria. In contrast, Ramitubulus exhibits only complete cross walls, unipolar growth and dichotomous branching. These features are difficult to reconcile with a cyanobacterial interpretation. They are, instead, more indicative of multicellular algae-like Cambrian Epiphyton. Thus, the Weng’an tubular microfossils constitute a disparate assemblage of cyanobacteria and algae, but none represents early Ediacaran animals.     

Early Pleistocene human mandible from Sima del Elefante (TE) cave site in Sierra de Atapuerca (Spain): a comparative morphological study

We present a detailed morphological comparative study of the hominin mandible ATE9-1 recovered in 2007 from the Sima del Elefante cave site in Sierra de Atapuerca, Burgos, northern Spain. Paleomagnetic analyses, biostratigraphical studies, and quantitative data obtained through nuclide cosmogenic methods, place this specimen in the Early Pleistocene (1.2-1.3 Ma). This finding, together with archaeological evidence from different European sites, suggests that Western Europe was colonised shortly after the first hominin expansion out of Africa around the Olduvai subchron. Our analysis of the ATE9-1 mandible includes a geometric morphometric analysis of the lower second premolar (LP₄), a combined and detailed external and internal assessment of ATE9-1 roots through CT and microCT techniques, as well as a comparative study of mandibular and other dental features. This analysis reveals some primitive Homo traits on the external aspect of the symphysis and the dentition shared with early African Homo and the Dmanisi hominins. In contrast, other mandibular traits on the internal aspect of the symphysis are derived with regard to African early Homo, indicating unexpectedly large departures from patterns observed in Africa. Reaching the most occidental part of the Eurasian continent implies that the first African emigrants had to cross narrow corridors and to overcome geographic barriers favouring genetic drift, long isolation periods, and adaptation to new climatic and seasonal conditions. Given these conditions and that we are dealing with a long time period, it is possible that one or more speciation events could have occurred in this extreme part of Eurasia during the Early Pleistocene, originating in the lineages represented by the Sima del Elefante-TE9 hominins and possibly by the Gran Dolina-TD6 hominins. In the absence of any additional evidence, we prefer not include the specimen ATE9-1 in any named taxon and refer to it as Homo sp.     

Dental tissue proportions and enamel thickness in Neandertal and modern human molars

The thickness of dental enamel is often discussed in paleoanthropological literature, particularly with regard to differences in growth, health, and diet between Neandertals and modern humans. Paleoanthropologists employ enamel thickness in paleodietary and taxonomic studies regarding earlier hominins, but variation in enamel thickness within the genus Homo has not been thoroughly explored despite its potential to discriminate species and its relevance to studies of growth and development. Radiographic two-dimensional studies indicate that Neandertal molar enamel is thin relative to the thick enamel of modern humans, although such methods have limited accuracy. Here we show that, measured via accurate high-resolution microtomographic imaging, Neandertal molar enamel is absolutely and relatively thinner than modern human enamel at most molar positions. However, this difference relates to the ratio of coronal dentine volume to total crown volume, rather than the quantity of enamel per se. The absolute volume of Neandertal molar enamel is similar to that of modern humans, but Neandertal enamel is deposited over a larger volume of coronal dentine, resulting in lower average (and relative) enamel thickness values. Sample sizes do not permit rigorous intragroup comparisons, but Neandertal molar tissue proportions evince less variation than the modern human sample. Differences in three- and two-dimensional enamel thickness data describing Neandertal molars may be explained by dimensional reduction. Although molar tissue proportions distinguish Neanderthals from recent Homo sapiens, additional study is necessary to assess trends in tissue proportions in the genus Homo throughout the Pleistocene.     

Functional morphology of Tethya species (Porifera): 2. Three-dimensional morphometrics on spicules and skeleton superstructures of T. minuta

The biomechanics of body contraction in Porifera is almost unknown, although sponge contraction has been observed already in ancient times. Some members of the genus Tethya represent the most contractile poriferan species. All of them show a highly ordered skeleton layout. Based on three main spicule types, functional units are assembled, termed skeleton superstructures here. Using synchrotron radiation based x-ray microtomography and quantitative image analysis with specially developed particle and structure recognition algorithms allowed us to perform spatial allocation and 3D-morphometric characterizations of single spicules and skeleton superstructures in T. minuta. We found and analyzed three skeleton superstructures in the investigated specimen: (1) 85 megasclere bundles, (2) a megaster sphere, composed by 16,646 oxyasters and (3) a pinacoderm–tylaster layer composed by micrasters. All three skeleton superstructures represent composite materials of siliceous spicules and extracellular matrix. From structure recognition we developed an abstracted mathematical model of the bundles and the sphere. In addition, we analyzed the megaster network interrelation topology and found a baso-apical linear symmetry axis for the megaster density inside the sphere. Based on our results, we propose a hypothetical biomechanical contraction model for T. minuta and T. wilhelma, in which the skeleton superstructures restrain physical stress generated by contraction in the tissue. While skeletal structures within the genus Tethya have been explained using R. Buckminster Fullers principle of tensegrity by other authors, we prefer material science based biomechanical approaches, to understand skeletal superstructures by referring to their composite material properties.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .