Tag Archives: CT

Cranial vault thickness in South African Australopiths

In a recent paper, Beaudet and colleagues analyze the cranial vault thickness of StW 578, a partial cranium of Australopithecus not yet assigned to a species. The authors explore the utility of cranial vault thickness and of the organization of the diploe and cortical tables as potential diagnostic criteria for hominin species. For that, they also analyze a comparative sample including other South African Late Pliocene-Early Pleistocene fossils, extant humans, and chimpanzee specimens. Fossils include specimens of Australopithecus and Paranthropus recovered from Sterkfontein, Swartkrans, and Makapansgat sites. Based on cranial landmarks, the authors defined homologous parasagittal and coronal sections on the CT scans, preferentially on the right hemisphere, which is better preserved in StW 578. The thickness of the diploe, the thickness of the inner and outer cortical tables, and the total thickness were measured automatically in various points sampled throughout the length of the sections. The proportion of each layer was computed by dividing the thickness by the surface area calculated between two successive points. Specimens that preserved only the left side were used for qualitative comparison. Results emphasize differences between Australopithecus and Paranthropus. The former genus tends to have thicker vaults, with a larger proportion of the diploic layer, while the latter tends to have thinner vaults, with a larger proportion of the inner and outer tables. The distribution of thickness also differs, as StW 578 and other Australopithecus crania from Sterkfontein display disproportionately thicker frontal and posterosuperior parietal regions, while Paranthropus (SK 46) and extant chimpanzees have thickest regions on cranial superstructures (supraorbital and occipital tori). As the authors suggest, thickening of the cranial vault in frontal and parietal regions needs further investigation, as to unveil a possible correlation between bone thickness and brain anatomy. Moreover, as the increase in thickness is associated with an increase in diploe proportions, variation in this layer might indicate physiological (thermoregulation) or biomechanical differences between Australopithecus and Paranthropus. In sum, cranial vault thickness patterns of StW 578 are equivalent to those of other specimens from Sterkfontein (StW 505 and Sts 71). The presence of a Paranthropus-like pattern in two of the three Mangapansgat specimens further indicates the presence of different morphs or species of Australopithecus in this site. This methodology and results provide a fine base for further studies on the taxonomic significance of the cranial vault thickness. The authors suggest beginning by including more Paranthropus specimens, and by evaluating chronological, geographic, and taxonomic variation.

 

Sofia Pedro

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Seasonal skull reduction

dechmann-et-al-2017Braincase shrinkage during winter was firstly described in shrews by Dehnel in 1949, and is known as the Dehnel’s Phenomenon. Recently, Dechmann et al. investigated the seasonal size variation in the skulls of shrews (Sorex araneus) and least weasels (Mustela nivalis). They measured skull length and braincase depth on specimens previously collected from a Polish National Park, sampling all seasons. Both species showed an initial juvenile growth until the first winter, followed by shrinkage until spring in the adults, and a subsequent re-grow on the second summer, though never reaching the initial size. Heat maps built from high resolution CT scans demonstrated that size changes also involved changes in shape and in bone thickness, with the thinnest skulls coinciding with the smallest braincase size. Interestingly, these patterns differed between sexes, especially in weasels as only males were observed to re-grow. Despite phylogenetically distant, both species have similar life histories, having short life spans and high metabolisms, and inhabiting an environment with seasonal fluctuation of resources availability. Winter shrinkage would reduce energetic requirements and prepare individuals for the harder conditions, and re-growth during the resources-abundant season would prepare the males for reproduction while females would allocate the energy into caring for the offspring.  The authors conclude these seasonal reversible size changes are genetically fixed and exclusive of animals with such life histories, as an adaptation to extreme environmental conditions. Future investigation shall clarify the potential drivers and consequences of this phenomenon, including how the variation in size affects brain size and reorganization.

Sofia Pedro


Hyena paleoneurology

hyenasA series of works by Sharleen T. Sakai’s group have correlated the proportions of the anterior endocranial region with social behaviour in hyenas. They found that in spotted hyenas (Crocuta crocuta), males have relatively larger anterior cerebrum than females. The relative volume of the anterior endocranial region is also significantly larger in this species when compared to other extant species of hyenas. The spotted hyenas are the most gregarious species, living in large clans, where females are dominant and philopatric, and males disperse and must adapt to the hierarchic system of a new clan. The anterior region of the hyena’s brain comprises mostly the frontal cortex, which mediates social behaviour. The authors hypothesize, in the light of the social brain hypothesis, that the development of the frontal region in this species, and particularly in males, might have been enhanced by the need for a larger behavioural flexibility in their complex social environment. More recently, Joan Madurell-Malapeira and his colleagues compared the endocasts of two extinct spotted hyenas (C. spelaea and C. ultima) with those of extant species. The fossil specimens have similar morphology to that of C. crocuta, but less developed anterior portion of their endocranium. The authors therefore propose this feature to be an autapomorphy of C. crocuta. Consequently, the social and foraging behaviour of these fossil species are presumably less specialized, and this might contradict some speculations about competition between hyenas and humans during Pleistocene.

Sofia Pedro


Notoungulata endocasts

Dozo and Martínez 2016Notoungulata is an extinct order of ungulates, endemic from South America. It has two main suborders: Toxodontia, including the large-bodied ungulates, and Typotheria. Researchers from Argentina have described the endocasts from two species of Notohippids, a family from the South American Oligocene that is included in the Toxodontia group. The endocasts from Rynchippus equinus and Eurygenium latirostris were virtually reconstructed from CT scans and compared to other fossil and extant ungulates. Both endocasts were similar in size and in their overall shape, proportions and sulci morphology. They fitted into the general “design” of the Toxodontia endocasts, which have the most complex surface within the Notoungulates, with pronounced telencephalic flexure, a developed Sylvian sulcus, and a bulging temporal lobe. These features are also similar to those displayed by the rabbit-like Typotheria group, although Notohippids had larger frontal region. In contrast, extinct and extant ungulates display a different endocast morphology, without prominent Sylvian and temporal regions. According to the authors, functional interpretations for the expansion of the frontal region and the Sylvian and temporal areas in the Notohippids can suggest an increase in the snout sensitivity and an auditory specialization, respectively.

Sofia Pedro