Tag Archives: endocasts
Bruner E. & Ogihara N. 2018. Surfin’ endocasts: the good and the bad on brain form. Palaentologia Electronica 21.1.1A: 1-10.
Since brain does not fossilize, brain endocast (i.e., replica of the inner surface of the braincase, Figure 1) constitutes the only direct evidence for reconstructing hominin brain evolution (Holloway, 1978; Holloway et al., 2004a). In this context, paleoneurology has suffered from strong limitations due to the fragmentary nature of the fossil record and the absence of any information regarding subcortical elements in extinct taxa. Additionally, variation in brain shape and organization (and in the corresponding endocast) is technically difficult to capture, as stated by Bruner (2017a, p. 64): “[…] the smooth and blurred geometry of the brain, its complex and complicated mechanisms, and its noticeable individual variability make any research associated with its morphology very entangled and difficult to develop within fixed methodological approaches.” An emblematic example might be the reluctance of paleoneurologists to consider the sulcal imprints visible on the endocranial surface because of the substantial uncertainties in describing such features in fossil specimens and related debates (e.g., the lunate sulcus in the Taung child’s endocast; Falk, 1980a, 2009, 2014; Holloway, 1981a; Holloway et al., 2004b). In 1987, Tobias even came to the conclusion that “The recognition of specific cerebral gyri and sulci from their impressions on an endocast is a taxing, often subjective and even invidious undertaking which arouses much argumentation” (p. 748) …
Encephalization quotients (EQ) have been extensively used to characterize brain evolution, but this univariate metric only includes information on relative size. Marugán-Lobón and his colleagues recently analysed the association between endocranial shape changes and EQ by applying geometric morphometrics to a sample of modern bird endocasts. A Principal Component Analysis accounting for phylogenetic history showed that the bird endocasts varied essentially in the relative expansion of the forebrain and in the degree of flexion of the braincase. The distribution of the specimens in the morphospace has a phylogenetic structure, with morphological affinity between close evolutionary clades, particularly the landbirds, which display larger forebrains. Size explains 10% of the shape variation. EQ accounts for changes in relative forebrain expansion, with larger EQs associated with larger forebrains. A second study was computed correcting for phylogeny, i.e. computing regression analyses on the phylogenetic independent contrasts of shape and size against EQ. When allometric and phylogenetic signals were removed, shape variation was mostly associated with the degree of flexion of the endocasts, and EQ was not significantly correlated with these morphological changes. The authors conclude that, excluding the general effect of size, EQ does not explain shape differences among birds’ endocasts. Therefore, other factors are probably responsible for brain variation in birds.
Notoungulata 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.
A team of researchers from Argentina has recently studied the endocranial morphology of Neotropical parrots. They reconstructed the endocasts from several species and conducted a morphological analysis to evaluate the previously proposed evolutionary history of these taxa. Their investigation supplies three main findings. First, these birds have higher than expected brain volumes for their body mass, and the authors suggest this might be associated with the evolution of cognitive abilities or their versatile behaviour. Second, two different morphotypes were distinguished according to the maximum width of the hemispheres: a more quadrangular or walnut-shaped brain and a more rounded brain shape. A reconstruction of the ancestral morphology is similar to the more rounded type. However, as the distribution of the two types across the species is heterogeneous, the authors hypothesize the walnut type might be the primitive for all the parrots, and the rounded type primitive for the Neotropical parrots.