Old Simons Bones

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Anapsid describes the condition where there are no temporal fenestrae and is characteristic of turtles and most basal amniotes Figure 1. Three basic patterns of fenestration are recognized. The diapsid condition has two temporal fenestrae, one above the other on either side of the skull Figure 1. These are the supratemporal upper or dorsal fenestra and the infratemporal lower or ventral fenestra.

This is the condition present in most reptiles. A single fenestra occurs as two variants. The euryapsid skull, characteristic of two reptile groups ichthyosaurs and plesiosaurs , bears a supratemporal fenestra Figure 1. Diagrammatic illustrations of the four main amniote skull patterns. The pattern of fenestration is defined by the typical configuration of bones that border the fenestra. The typical configurations of bones for the skull types are illustrated in Figure 1. In the synapsid condition the dorsal border of the fenestra is formed mainly by the squamosal and postorbital bones, although the parietal may occasionally participate, whereas the ventral border is formed mainly by the squamosal and jugal bones , with the quadratojugal bone occasionally contributing.

The infratemporal fenestra of the diapsid skull is bordered by the jugal, squamosal, and postorbital bones, with the quadratojugal occasionally participating. The supratemporal fenestra is bordered by the postorbital, squamosal, parietal, and, in many cases, the postfrontal bones. Two bony bars, temporal bars or arcades , are clearly defined in the diapsid skull, a ventral bar formed mainly by the jugal and quadratojugal bones, and a dorsal bar, between the fenestrae, formed by the postorbital and squamosal bones.

The fenestra of euryapsid skulls is bordered usually by the parietal, postfrontal, postorbital, and squamosal bones, with the last two meeting ventrally below the fenestra. From these basic patterns, several specializations have evolved, as discussed later.

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For much of the past century, the classification of amniotes closely reflected fenestration; and hence Anapsida, Diapsida, Euryapsida, and Synapsida were used as formal names for amniote radiations. More recently, however, we have realized that while the pattern of fenestration does broadly reflect amniote evolution, it is not tied as strictly to phylogeny as was once presumed.

At least two of these groups, Diapsida includes archosauromorphs and lepidosauromorphs and Synapsida includes mammals , are still considered monophyletic, but for the former we recognize that at least the earliest basal members of the clade had anapsid skulls. Within Diapsida a variety of specializations evolved.

Among those that display a fully diapsid pattern are tuataras Sphenodon and crocodylians. Birds, lizards, and snakes, however, have tended to lose one or both temporal bars. This has decoupled the posterior part of the skull, allowing the potential for considerable flexibility among the functional regions of the skull. In general, lizards have lost the lower temporal bar and snakes both the lower and upper temporal bars.

At this point, we must elaborate on the presence of the classic diapsid condition in Sphenodon. We noted earlier that Sphenodon possesses the lower temporal bar. For many years, this condition was interpreted as the retention of an ancestral diapsid condition, and so Sphenodon was long designated as a living fossil.

However, recent work has demonstrated that all lepidosaurs rhynchocephalians and squamates inherited a skull without a lower temporal bar; that is, it is the ancestral condition for this clade. Birds have lost the upper temporal bar so the infratemporal and supratemporal fenestrae have merged into a single large opening.

Further, they have lost the bony bar posterior to the orbit, merging the orbit and the fenestrae. Among synapsids, in mammals the fenestra increases in size and the bony bar behind the orbit generally disappeared so that orbit and fenestra merge. Although this resembles the condition in birds, the morphologies are convergent. Euryapsida is no longer considered a valid name. It was applied to ichthyosaurs and plesiosaurs, but we now recognize that the euryapsid pattern evolved independently in these groups from the diapsid condition, and thus that these groups and relatives, such as placodonts are diapsids.

Anapsida was applied to the more basal amniotes, as the anapsid skull is the basal and nearly ubiquitous type among early tetrapods.

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Recent phylogenetic analyses have revealed, however, that some early amniotes with anapsid skulls are more closely related to Diapsida than to other anapsid-skulled early amniotes. Anapsida is still used by some authors e. More commonly, this clade is termed Parareptilia.

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As noted earlier, the phylogenetic position of turtles is controversial and the composition of Parareptilia with or without turtles varies accordingly. Cavia porcellus , the scientific name for the domesticated guinea pig, was conferred upon the species by the naturalist Erxleben in the late 18 th century. The taxonomic outline for domesticated guinea pigs is as follows Wagner, :.

Guinea pigs are rodents within the hystricomorpha porcupine-like rodents suborder while the suborders sciuromorpha squirrel-like rodents and myomorpha rat-like rodents contain the other rodent species. An enlarged infraorbital canal is also present in many hystricomorph rodents, as with the capybara. Members of the Caviidae family all have four digits on their forefeet and three on their hindfeet, with the soles of the feet being hairless Harkness et al. Guinea pigs and others from the Cavia genus have one pair of mammaea and a vestigial tail Harkness et al.

While the tendency to gnaw and the presence of two front incisors one set of hypsodontic incisors with the enamel being restricted to the buccal surface were the primary reasons behind the guinea pig being classified as a rodent, several researchers have recently tried to re-classify the species outside the order Rodentia based on molecular studies of evolutionary changes to key proteins Frye and Hedges, Accelerated evolutionary rates of change in insulin, as demonstrated by deviating amino acid sequences, led the guinea pig to have an insulin protein that deviates strongly from other mammals starting a controversy Beintema and Campagne, ; Pritt, Quick refutation of the proposed reclassification of the guinea pig came with the findings that several nucleotide sequences unique and specific to rodents are also found in guinea pigs, but the controversy remained alive for several years with published research supporting both sides Cao et al.

Konno and co-authors suggested that depending on which sequences were studied, amino acid versus nucleotide, different phylogenetic position non-rodent versus rodent, respectively could be argued Konno et al. The controversy was essentially put to rest just a few years ago based on an eventual preponderance of data supporting the existing classification scheme.

The guinea pig now remains in its historical classification position Carleton and Musser, If one compares skulls of New World monkeys with those of Old World monkeys, one can see that the former usually have comparatively smaller facial skeletons than do Old World monkeys see Figure 5. There is one exception to this rule in genus Alouatta, the howler monkey in which the skull morphology is influenced by a substantial enlargement of the hyoid bone of the throat Figure 5. The lower jaw is unusually deep, and the mandibular symphysis is steeply oblique.

This condition also occurs in the skulls of old male orangutans Hofer, In addition, the foramen magnum of howlers opens far more backward compared with any other South American primates.

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Unlike other platyrrhine monkeys, the posterior portion of the skull the occipital behind the mandibular ascending ramus is shorter in howlers than in other monkeys. The enlargement of the hyoid is much greater in male than in female howlers. Figure 5. Skull base of an anthropoid genus Macaca showing basicranial foramina. Skull, mandible, and inflated hyoid of a male howler monkey, genus Alouatta. Hyoid seen from left side and from posterior.

Petrosal bones of some of the small-bodied South American monkeys have comparatively inflated, large bullae. In the temporal fossa of New World monkeys, there is usually a suture between the jugal and parietal bones, as is also frequently the case in the colobines of the Old World Vogel, The genus Saimiri, the squirrel monkey, has a rather peculiar skull.


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Saimiri is the smallest cebid monkey, and whereas the facial skeleton is comparatively small, its relatively large brain dominates conformation of the skull. The foramen magnum is shifted far forward toward the center of the skull. The occipital portion of the skull projects much further backward than in other monkeys as well, and consequently the foramen projects straight downward.

The result of this combination of features is that the foramen magnum is situated relatively farther forward toward the front of the skull compared even with humans. In humans, the central position of the foramen magnum can be explained in part by their unique bipedal, upright locomotion. This is not so in the Samiri , a quadrupedal runner Biegert, Here we have a perfect example of two similar morphological traits that have arisen for very different reasons in these two types of primates.

Additionally, the likeness of the position of the foramen magnum in Tarsius , Saimiri, and Homo sapiens shows that the central position need not, as was once thought, be an indicator of upright walking or even of vertical clinging and leaping.

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In Saimiri , the comparatively large size of the brain is partly related to the fact that Saimiri is the smallest cebid monkey and that its brain is relatively complex. The squirrel monkey's brain, like that of all the Cebidae, is more derived than that of marmosets or tamarins, the other South American monkeys, which are of similar body size to Saimiri. Although squirrel monkeys have large brains, the size of the masticatory and olfactory apparatus of these small monkeys is unusually small, even in direct proportionality to the larger cebids.

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Thus, the unique skull morphology of squirrel monkeys appears to be a compromise between a small facial skeleton and a relatively large braincase. There is another unique feature of the Saimiri skull: all squirrel monkeys have a comparatively large hole in the bony intraorbital ethmoid wall Maier, This hole usually measures more than 1.

As yet, there is no clear understanding of this structure, but its presence in a late Oligocene fossil ceboid, Dolichopithecus, makes it of ancient origin. The hole certainly cannot be a response to eye size, because squirrel monkeys are diurnal primates and do not have enlarged eyeballs. In contrast, in the night monkey Aotus, with its enlarged eyes, the hole does not occur.

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Correlated with this feature of Saimiri , the interorbital breadth is much reduced. Compared with braincase size, the facial part of the skull, is larger in catarrhines than in platyrrhines. Among the catarrhines, the snout is frequently long and prominent, a feature that is especially marked in those forms—such as the baboons and great apes—that are comparatively large.

In the case of these primates, the prominent anterior part of the skull, also called the snout or rostrum, is not enlarged in correlation with the expanded olfactory region and sense of smell as it is, for example, in the dog family. Rather, its size seems to be related to the large masticatory apparatus typical of the Pongidae. In the case of the Cercopithecidae, large snouts have been correlated with both the enormously enlarged canines and the lengthened tooth row and associated chewing musculature in baboons genera Papio, Madrillus, and Theropithecus; see Figure 5.

Side view of the skull of a baboon genus Papio showing the various bones. This phenomenon is partly the result of their allometric increase in body size relative to their ancestors. With increasing overall body bulk, the masticatory apparatus also increases, but the proportionate size of the brain does not increase at the same rate Biegert, ; Martin, Among Old World higher primates, the petrosal is not expanded into a balloonlike protrusion as it is among Prosimii and New World primates.