Primates are unique among mammals for having a brain much larger than expected for body size. An important aim in paleoneurology is understanding how cerebral structures reorganized to accomodate primate cerebral expansion. The brain comprises only soft-tissue and does not fossilize so paleoneurologists rely on endocasts, either physical or digital molds of the cranial cavity, to estimate the macro-anatomy of the brain. Continuing computational advances and powerful imaging techniques have allowed the generation of increasingly higher-resolution digital endocasts. Gonzales et al. (2015) generated a high-resolution endocast of the 15 Myr-old fossil cercopithecine Victoriapithecus macinnesi using micro-CT scans. By using computational methods, taphonomic distortion was corrected and a new endocranial volume (ECV) of 35.6 cm3 reported for Victoriapithecus which is much smaller than the previous value 54 cm3. This new, smaller ECV places Victoriapithecus within the range of extant strepsirrhines but outside the range expected of extant and fossil cercopithecoids including the 32 Myr-old fossil species Aegyptopithecus zeuxis which had an ECV within the expected range for fossil cercopithecoids.
Despite Victoriapithecus exhibiting a very small ECV and falling below the range for extant cercopithecoids, the fossil does exhibit the ‘frog-shaped’ sulcal pattern shared only among cercopithecines. This sulcal pattern suggests Victoriapithecus is a cercopithecine, the ‘frog-shaped’ sulcal pattern is such a diagnostic trait that it is not shared by the leaf-eating colobines but only present in cercopithecines. The olfactory bulbs in Victoriapithecus are unusually large relative to the small ECV. Large olfactory bulbs are present in extant strepsirrhines and the fossil catarrhine Aegyptopithecus zeuxis but reduced in all extant and fossil cercopithecoids and hominoids. The presence of small olfactory bulbs in the 18 Myr-old hominoid Proconsul versus the large bulbs in Victoriapithecus suggested olfactory bulb reduction may have evolved independently in both cercopithecoids and hominoids.
Harrington et al. (2016) compared digital endocasts generated from micro-CT of three adapiform fossil primates including the 48 Myr-old Notharctus tenebrosus, 47 Myr-old Smilodectes gracilis and 45 Myr-old Adapis parisiensis. Results of endocranial volume (ECV) were consistent with other studies revealing an ECV of 7.6 cm3 for Notharctus, an ECV of 8.3 cm3 for Smilodectes while Adapis had an ECV of 8.8 cm3. The sulcal morphology of these adapiforms was also consistent with previous studies showing the defining feature of the primate brain, the Sylvian sulcus, is species-specific in these adapiforms. The Sylvian sulcus is well-defined in Adapis, occurs only as a shallow depression in Notharctus but is entirely lacking in Smilodectes. The absence of the Sylvian sulcus in Smilodectes is not understood but as it is absent in other mammals, this may represent a retained ancestral trait from before the divergence of primates from other mammals.
The cerebral organization of Notharctus and Smilodectes showed both possessed larger temporal and occipital lobes relative to brain size with smaller olfactory bulbs and frontal lobes. This trend might indicate cerebral reorganization favoring larger visual-auditory structures located in the temporal-occipital regions of the brain versus smaller visual-olfactory structures in the frontal region. The olfactory bulbs of these adapiforms were small and blunt relative to endocranial volume and predicted body mass but uniquely, Adapis parisiensis had the largest olfactory bulbs, placing it within the range of extant strepsirrhines. These studies reveal how little is understood about primate paleoneurology and the evolutionary trends of different primate lineages with implications for the human fossil record.
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) …
[keep on reading this Opinion Article by Amélie Beaudet in Frontiers in Human Neuroscience, published in a special issue dedicated to Language, skull, and brain]
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.
Two new endocasts on AJPA:
Maba (Asia, 300-130 ky) and Buia (Africa, 1 My).
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.