Obliteration study of lambdatic and obelionic region sutures in ruminant , carnivores and hominids

The morphology of Orce cranial fragment VM-0 is contrasted with the fronto-parietal region in artiodactyls, and the obelionic region in carnivores and primates including hominids. Sutural development at obelion is compared in those taxa throughout the growth period up to the onset of sutural obliteration, and ontogenetic differences between non-primates and primates lead us to conclude that the configuration in VM-0 more resembles that found in hominids than in artiodactyls or carnivores. Moreover, cranial capacity for VM-0 is estimated at >470cm3, comparable to Plio-Pleistocene hominids, but greater than in young equids.

New anatomical research presented here is based on changes observed at the obelionic region during growth in humans and other large mammals and differential growth here during postnatal ontogenesis is found to be a distinctively human characteristic.Furthermore, cranial capacity in VM-0 is demonstrably closer to human rather than other mammalian values.

Materials, methods and theoretical considerations
Sources and inventory numbers of juvenile, infant and newborn specimens principally are given in table 1. Biological age is based on museum data-bases, supplemented in some cases by our observations on dental eruption.Cranial capacity was determined directly using small gramineae seeds whose volume was subsequently measured in a graduated cylinder."Excel" was used to draw up graphs .Non-human specimens (table 1) are from Barcelona's Zoology Museum (MZB), the Autonomous University of Barcelona's (UAB), Veterinary Faculty (not included in table 1), the Laboratory of Comparative Anatomy in Paris (LAP), the University of Hamburg (UH).Human remains are from the Autonomous University of Barcelona's Anthropology Unit.We gratefully acknowledge data provided by the Conservator of Barcelona's Zoological, Dr. Filella.Tomographs were carried out by Dr. X. Roura of the Autonomous University of Barcelona's Veterinary Faculty.
VM-0 has two sutures that come together at a point, as well as very wide longitudinal and transverse arcs (figs.1, 2, 4).Broad-ly speaking, similar features are seen in the bregmatic and fronto-parietal region of artiodactyl skulls and the obelionic region of hominid, carnivore and equid skulls.With regard to the former, if the point where the VM-0 sutures come together is presumed to be bregma, then VM-0 has longitudinal and transverse arcs that put it in a category on its own.With regard to obelionic comparisons, matters are complicated by the need to take into consideration, not only the biological age at which obliteration occurs in lambdoid and occipital sutures (and, for contrast, coronal sutures), but also, both increases in cranial capacity during ontogenesis, and anatomical changes in the external surface at obelion during that period, in taxa such as equids, bovids, cervids, carnivores, and lagre primates (in cluding hominids).

Anatomical description of VM-0
The cranial fragment VM-0 (fig. 1) is a small portion of a cranial vault, with a maximum coronal width of 76 mm and sagittal length of 80 mm, that includes lambda and comprises two posterior fragments of the right and left parietal bones (the right fragment being almost twice the size of the left), part of the sagittal suture (visible on both external and internal bony tables), and the apex of the occipital squame (separated from the parietal fragments by parts of clearly recognizable right and left lambdoid sutures).The surface of the outer table is somewhat eroded, like many other fossils from Venta Micena, and a few fracture lines are visible, though there are no signs of bony regeneration.VM-0 is not deformed (except perhaps beyond the most anterior fracture, although any displacement there is of trivial significance) (fig.3).The specimen provides enough anatomical elements to enable reliable conclusions to be drawn when compared to other mammals.In fact, none of the studies of Agustí & Moyà (1987), Moyà & Agustí (1989), Moyà & Köhler (1997) or Martinez-Navarro (2002) claim that the specimen is deformed.

Morphology of the external aspect of the parietal and bregmatic regions of the ruminant
Male bovid horns and cervid antlers are associated with fusion of the parietal bones such that obliteration of the sagittal suture takes place at birth or soon afterwards (e.g.immature type specimens of Elaphurus davidianus MZB 94-1062, Rupicapra rupicapra MZB 98-1330, Bison bison MZB 94-0667 and Bos taurus MZB 94-0477); obliteration of the sagittal suture had commenced proximally and terminated distally in a young female barely a few months old of Rupicapra rupicapra (MZB 98-1330), and in all cases studied commencement of obliteration in coronal and lambdoid sutures followed commencement of obliteration in the sagittal suture.A more or less developed crest appears by only a few months of age in cervid and bovid parietals (fig.4).
The complexity of the indentations of the coronal and interfrontal sutures is variable and the angle that these form with reference to bregma also varies, between 85 and 145 degrees (Gibert et al., 1989a, table 5).In terms of its complexity, the interfrontal suture can be divided into a shorter part (less than half its total length) which offers a complexity similar to that of the coronal suture (fig.1; cf.Gibert et al., 1989a, figs. 57 and 58), and a longer part which is straight.Following attainment of the juvenile stage in cervids the interfrontal suture is aligned along a visible crest (fig.4).Supraorbital foramina lie medially on the frontal bones and give rise to anteriorly-directed supraorbital grooves.Medial to the supraorbital foramina the frontal bones lose their lateral convexity and become flattened in the coronal plane, tending to become depressed where the complexity of the interfrontal suture gives way to straightness (fig.4).At bregma the frontal coronal or transverse arc is strongly curved, though between 5 and 10 mm from bregma it changes its form such that curvature is maintained only in the midline and on either side of the interfrontal suture, undergoing an inflexion that renders it almost flat  towards the extremes (figs.1, 4).The transverse curvature near bregma is inflected at the level of cervid and bovid suporaorbital foramina.

Morphology of the internal table at the bregmatic region in bovids and cervids
Sutures on the internal table of bovid and cervid skulls become obliterated near bregma before they do so laterally, and their pattern tends to become simpler.Bovid frontals display well-formed sinuses throughout the bone, even in infants, most particularly in horned genera such as Bos, Bubalus, Bison and Tragelaphus.In juveniles and adults of Kobus and Boselaphus (whose females lack horns) major sinuses lie posterior to the orbital foramina (figs.1, 4).Juvenile and adult cervid frontals have sinuses in the same region as bovids.In bovids grooves are visible on the internal table beneath the interfrontal suture (fig.4) whilst in cervids there is marked thickening here in both juveniles and adults.

Comparison with modern artiodactyls
The bregmatic region is thick in modern artiodactyls skulls, including infant specimens (table 2).The thickness of VM-0 is commensurable only with infant cervids, whilst being differentiated from them by VM-0's larger dimensions (fig.5).Any similarity between the external table of VM-0 and artiodactyls is confined to the form of coronal and interfrontal sutures, which might otherwise be confused with VM-0's lambdoid and sagittal sutures (cf.Gibert et al., 1989a, b).As can readily be seen from figures 1 and 5,6 the morphology of VM-0 is not comparable with the bregmatic region of bovids or cervids because their frontal regions, in both juvenile and adult specimens, exhibit significant alterations of curvature at the level of the supraorbital foramina, from 5 to 10 mm away from bregma in the coronal plance.Owing to presence of sinuses in infant, juvenile and adult specimens of Bos, Bubalus and Bison, VM-0 cannot be compared to those genera, nor probably with their fossil ancestors either.Neither can VM-0 be compared with bovids such as Bosela-  The anatomy of the external table of artiodactyls frontal bones is markedly different from that presented by VM-0, as is demonstrated by anatomical comparisons using computerized tomography.The presence of sinuses in the frontals of infants, juveniles and adults of some bovids, and in juvenile and adult cervids, excludes VM-0 from these groups.
The thickness of the bregmatic region in bovids, even in infant specimens, is much greater than that of VM-0.Infant cervids exhibit a similar thickness to that of VM-0, but their transverse (coronal) and longitudinal (sagittal) curvatures are markedly different, as are the differences in size
In equids sagittal sutural obliteration begins at around 8-9 months.It was still not obliterated on the external cranial surface in 2-to-5-month-old specimens (UH 7476, 5444, 6933), but a 9-monthold specimen of Equus burchelli (MZB 94-1160) shows obliteration of the middle part of the suture and onset of external sagittal crest formation.As in carnivores, coronal sutural obliteration follows that of the sagittal suture.In some specimens (e.g.MZB 94-1160) sagittal sutural obliteration begins earlier on the inner than on the outer surface of the skull.At obelion, the order of closure in equids begins with the sagittal suture, followed by the interfrontal suture, then by the coronal sutures, and finally the lambdoid sutures.In carnivores, sagittal sutural closure occurs first, but we have not been able to determine the subsequent sequence.

Primates
In Primates, unlike other mammals, sagittal sutural closure takes place very late.We have never seen it obliterated in any specimens designated as "young" in museum catalogues.It was still open in a 5-year-old Colobus gereza (MZB 98-0014), an allegedly "young" Gorilla gorilla (MZB 82-547) with a cranial capacity of 400 cm 3 well on the way to that of adults (475 cm 3 ), and similarly "young" specimens of Colobus torquatus (MZB 94-1363), Mandrillus sphinx (MZB 96-0411) and Papio leucocephalus (MZB 91-0316).Nowadays, in human beings sagittal sutural closure can begin at age 13 even though cerebral growth may continue till age 20.

Increase in brain volume during growth
Primates differ, then, from bovids, cervids, carnivores and equids, in all of which sagittal sutural obliteration has begun by 7-9 months after birth.Sutural closure sets severe limits to brain growth, whereas retardation of closure may allow it to con-tinue growing.There is a fundamental difference here between non-Primates and Primates, and between non-human primates and humans.In nonprimates mammals brain mass is proportional to body mass, whereas in hominids there is an allometrical relation.Comparison of body weight (kg) with brain volume (cm 3 ) in adults of P. tigris, E. burchelli, G. gorilla and H. sapiens sapiens (fig. 8) shows the well-known disproportionate size, in our species, of the brain in relation to the body.When we consider those relationships at the time of sagittal sutural closure (fig.9), the difference is particularly marked; thus whereas the suture closes earliest in P. tigris, followed a few weeks later by the heavier E. burchelli, in Primates it is much delayed, and   13c).In short, the time elapsed between birth and sagittal sutural closure is proportional to brain size, and the longer the time, the greater is its size.

The VM-0 sagittal suture
There is general agreement in the published literature that VM-0 corresponds to an obelionic region of a mammalian skull in which the sagittal suture has not undergone obliteration (Agustí & Moyà, 1987;Campillo, 1989Campillo, , 1999;;Gibert, 1986;Campillo & Gibert, 1996;Gibert et al., 1983Gibert et al., , 1989aGibert et al., -c, 1998aGibert et al., , 1999a, b;, b;Moyà & Agustí, 1989).From considerations explained above, comparison is admissible only with bovids and cervids under 2 months old or carnivores and equids under 9 months old.However, comparison with carnivores and equids shows VM-0 to have greater dimensions and quite different arcs around obelion (figs.1, 7); corroborating earlier comparisons with equids: Gibert et al., 1998aGibert et al., , b, 1999b)).The comparisons highlight a brain size for VM-0 well in excess of the 470 cm 3 of 9-month-old equids when their sagittal sutural closure begins or an adult equid value of 500 cm 3 .Taking into account infantile features and thinness of VM-0 (Campillo, 1989;Gibert et al., 1989a-c) it can be deduced that growth was incomplete.Notwithstanding allegations to the contrary (Agutí & Moyà, 1987, Moyà & Agustí, 1989), VM-0 does not present a partly closed coronal suture; indeed, were there to have been a partly closed equid coronal suture the sagittal suture would have been utterly and completely obliterated, which is obviously not the case!

Fig. 3 .
Fig. 3.-Longitudinal tomography of VM-0 into lambda and two milimeter, lef and right.Broken in diastal portion is interpreted bay Moyá and Agusti as coronal suture.

Fig
Fig. 10.-Comparison of body weight (kg) and brain volume (cm 3 ) with months after birth between P. Tigris and E. Burchelli.

Table 2 .-Skull thickness of different specimens and its sex and age
A) Maximum-minimum thickness obelionic region.B) Thickness at bregma.The thickness is expressed in mm.