what animals have a forward foramen magnum

The forward shift of the foramen magnum is apparent in bipedal hominins, including modern humans, Australopithecus africanus

, and Paranthropus boisei

Paranthropus boisei

Paranthropus boisei or Australopithecus boisei was an early hominin, described as the largest of the genus Paranthropus. It lived in Eastern Africa during the Pleistocene epoch from about 2.4 until about 1.4 million years ago.

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. This common feature of bipedal hominins is the driving argument used by Michel Brunet that Sahelanthropus tchadensis was also bipedal, and may be the earliest known bipedal ape.

Full Answer

What is the significance of the foramen magnum in mammals?

Foramen magnum. Another landmark is the basion located at the midpoint on the anterior margin of the foramen magnum. The foramen magnum is a very important feature in bipedal mammals. One of the attributes of a bipedal animal’s foramen magnum is a forward shift of the anterior border; this is caused by the shortening of the cranial base.

Do bipedal marsupials and rodents have different foramina Magna?

Our comparative data reveal that bipedal marsupials and rodents have foramina magna that are more anteriorly located than those of quadrupedal close relatives. The foramen magnum is also situated more anteriorly in orthograde strepsirrhines than in pronograde or antipronograde strepsirrhines.

Which primates exhibit the most anteriorly positioned foramen Magna?

The foramen magnum is also situated more anteriorly in orthograde strepsirrhines than in pronograde or antipronograde strepsirrhines. Among the primates sampled, humans exhibit the most anteriorly positioned foramina magna.

What causes the anterior border of the foramen magnum to shift?

One of the attributes of a bipedal animal’s foramen magnum is a forward shift of the anterior border; this is caused by the shortening of the cranial base. Studies on the foramen magnum position have shown a connection to the functional influences of both posture and locomotion.

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What animals have a foramen magnum?

According to the findings, a foramen magnum positioned toward the base of the skull is found not only in humans, but in other habitually bipedal mammals as well. Kangaroos, kangaroo rats and jerboas all have a more forward-shifted foramen magnum compared with their quadrupedal (four-legged walking) close relatives.

Where is the foramen magnum located in animals?

The foramen magnum is the largest foramen of the skull. It is located in the most inferior portion of the cranial fossa as a part of the occipital bone.

Do humans have foramen magnum forward?

Compared with other primates, the large hole at the base of the human skull where the spinal cord passes through, known as the foramen magnum, is shifted forward.

Do gorillas have foramen magnum?

Broca (1872) refined the angular measurement of foramen magnum position used by Daubenton (1764) to demonstrate that humans have more anteriorly positioned foramina magna than chimpanzees, gorillas, orangutans, gibbons, and an unspecified array of guenons, langurs, and papionins.

Where is the foramen magnum on the chimpanzee skull?

In apes the foramen magnum lies well behind (posterior) of the bitympanic line, posterior of a relatively long basioccipital. In addition to being more posteriorly positioned, the foramen magnum in apes is more vertically oriented (opening backwards and downwards, rather than directly downwards).

What type of skull is found in mammals?

Dicondylic skulls are known as the skull, which is connected to the body with two articulatory condyles. It is present in mammals and amphibians. The skull has two occipital condyles in amphibia, and the skull is bound by the first vertebra of the vertebral column, i.e., atlas, with the aid of these two condyles.

How does the position of the foramen magnum correlate to how an animal walks?

Unlike humans, chimpanzees typically move on four limbs, so their foramen magnum sits near the back of the skull. But in upright-walking humans, the hole is at the bottom. This lets the head balance on the spine like a golf ball balances on a tee, Ruth said.

Why did the foramen magnum move forward?

One of the attributes of a biped's foramen magnum is a forward shift of the anterior border of the cerebellar tentorium; this is caused by the shortening of the cranial base. Studies on the foramen magnum position have shown a connection to the functional influences of both posture and locomotion.

Are there any bipedal animals?

Humans, birds, many lizards and (at their highest speeds) cockroaches run bipedally. Kangaroos, some rodents and many birds hop bipedally, and jerboas and crows use a skipping gait. This paper deals only with walking and running bipeds. Chimpanzees walk with their knees bent and their backs sloping forward.

Do quadrupeds have a foramen magnum?

In a quadruped, the spinal column also runs parallel to the ground so the foramen magnum is more dorsally placed (i.e., toward the back of the cranium). In a bidped, the spinal column runs perpendicular to the mandible and the ground. The foramen magnum is located more inferiorly (more on the bottom of the cranium).

What is the largest foramen in the skeleton?

The foramen magnum is the largest foramen of the skull. It is located in the most inferior portion of the cranial fossa as a part of the occipital bone.

Do humans have bipedal?

Humans are the only primates who are normally biped, due to an extra curve in the spine which stabilizes the upright position, as well as shorter arms relative to the legs than is the case for the nonhuman great apes.

How does the location of the foramen magnum indicate if a species was bipedal?

If the foramen magnum indicates the position of the spine in relation to the head, and therefore whether the creature was bipedal or moved about some other way, then the position of the opening might indicate when our ancestors developed the upright, bipedal posture so often taken to be the hallmark of humanity.

How does the position of the foramen magnum correlate to how an animal walks?

Unlike humans, chimpanzees typically move on four limbs, so their foramen magnum sits near the back of the skull. But in upright-walking humans, the hole is at the bottom. This lets the head balance on the spine like a golf ball balances on a tee, Ruth said.

Where is the foramen magnum located on a quadruped?

In a quadruped, the spinal column also runs parallel to the ground so the foramen magnum is more dorsally placed (i.e., toward the back of the cranium). In a bidped, the spinal column runs perpendicular to the mandible and the ground. The foramen magnum is located more inferiorly (more on the bottom of the cranium).

What is foramen magnum in anatomy?

human skull The foramen magnum, the opening through which the brain and the spinal cord make connection, is in the lowest part of the fossa. Between its forward margin and the base of the dorsum sellae is a broad, smooth, bony surface called the clivus (Latin for “hill”).…

What is the foramen magnum?

An anteriorly positioned foramen magnum is a basicranial feature shared by multiple clades of bipedal mammals. Humans, bipedal rodents, and bipedal marsupials all have more anteriorly positioned foramina magna than their quadrupedal close relatives. Orthograde strepsirrhines also have more anteriorly positioned foramina magna than pronograde or antipronograde strepsirrhines. These comparative findings suggest that habitual bipedalism and the adoption of upright trunk postures are both associated with rostral shifts in the position of the foramen magnum in mammals. Nevertheless, humans exhibit more anteriorly positioned foramina magna than all other primates considered in this analysis, including highly orthograde taxa like Hylobates and Propithecus. The unique position of the foramen magnum in humans compared with other living orthograde primates, as well as the general association between bipedalism and an anteriorly positioned foramen magnum in mammals, suggest that foramen magnum position may be used to identify bipedal adaptations in fossil hominins. Accordingly, the anteriorly positioned foramina magna of Sahelanthropus and Ardipithecus compared with extant hominoids provide strong evidence that both fossil genera were habitually bipedal.

Why is the foramen magnum anteriorly positioned?

The anterior position of the human foramen magnum is often explained as an adaptation for maintaining balance of the head atop the cervical vertebral column during bipedalism and the assumption of orthograde trunk postures. Accordingly, the relative placement of the foramen magnum on the basicranium has been used to infer bipedal locomotion and hominin status for a number of Mio-Pliocene fossil taxa. Nonetheless, previous studies have struggled to validate the functional link between foramen magnum position and bipedal locomotion. Here, we test the hypothesis that an anteriorly positioned foramen magnum is related to bipedalism through a comparison of basicranial anatomy between bipeds and quadrupeds from three mammalian clades: marsupials, rodents and primates. Additionally, we examine whether strepsirrhine primates that habitually assume orthograde trunk postures exhibit more anteriorly positioned foramina magna compared with non-orthograde strepsirrhines. Our comparative data reveal that bipedal marsupials and rodents have foramina magna that are more anteriorly located than those of quadrupedal close relatives. The foramen magnum is also situated more anteriorly in orthograde strepsirrhines than in pronograde or antipronograde strepsirrhines. Among the primates sampled, humans exhibit the most anteriorly positioned foramina magna. The results of this analysis support the utility of foramen magnum position as an indicator of bipedal locomotion in fossil hominins.

What landmarks are used to measure the position of the foramen magnum?

The three bony landmarks that we used to measure relative basion position in this analysis (posterior molar, posterior hard palate, and anterior temporal fossa) were chosen because these points could be repeatedly identified and measured with confidence across all taxa. Although many earlier studies of foramen magnum position have quantified the location of basion relative to the bicarotid or biporion chords (e.g., Dean and Wood, 1981, White et al., 1994, Schaefer, 1999, Brunet et al., 2002, Ahern, 2005, Suwa et al., 2009 ), the morphological diversity of our comparative sample required the use of alternate osteological landmarks. For example, in dipodid rodents the carotid artery passes through a groove along the medial aspect of the auditory bulla that is contiguous with the jugular foramen ( Hill, 1935 ). As a result, distinctive aspects of the carotid foramen (e.g., the center of the foramen as determined by the intersection of anteroposterior and mediolateral diameters ( Dean and Wood, 1981 ); the medial-most point on the foramen ( Ahern, 2005 )) are not easily recognized in these rodent specimens. Similarly, prenatal involution of the internal carotid system in cheirogaleid and lorisiform strepsirrhines leads to the loss of a readily visible carotid foramen in adult specimens ( MacPhee, 1981, MacPhee and Cartmill, 1986 ).

How to measure cranial size?

To compare the distances from basion to the posterior molar, posterior hard palate and anterior temporal fossa across a wide range of taxa that vary in body size, we calculated cranial size as the geometric mean of cranial length and width. This measure of cranial size scales isometrically and is highly correlated with body mass across mammals ( Muchlinski, 2010 ). We then divided the linear distance from basion to each anterior landmark by cranial size to create a ratio. These size-adjusted ratios are henceforth referred to as ‘molar ratio’ (distance from posterior molar to basion line/cranial size), ‘temporal fossa ratio’ (distance from anterior temporal fossa to basion line/cranial size), and ‘palate ratio’ (distance from posterior hard palate to basion line/cranial size). Low ratio values indicate that the foramen magnum is relatively anteriorly positioned, and high ratio values indicate that the foramen magnum is relatively posteriorly positioned.

What is the position of the foramen magnum?

The anterior position of the foramen magnum in humans compared with other primates has been a subject of discussion among comparative anatomists and anthropologists for nearly 250 years. As early as the mid-eighteenth century, Daubenton (1764) documented that the foramina magna of humans are more anteriorly situated on the cranial base than those of chimpanzees, lemurs, dogs, horses, or reptiles. Broca (1872) refined the angular measurement of foramen magnum position used by Daubenton (1764) to demonstrate that humans have more anteriorly positioned foramina magna than chimpanzees, gorillas, orangutans, gibbons, and an unspecified array of guenons, langurs, and papionins. These findings were reinforced by Topinard (1890), who applied the methods of both Daubenton (1764) and Broca (1872) to a comparative sample of 25 humans and 38 other anthropoid primates. Subsequent analyses have added further weight to the results of these early comparative studies based on new measurement techniques, better taxonomic sampling of primates, and larger interspecific sample sizes ( Bolk, 1909, Şenyürek, 1938, Schultz, 1942, Ashton and Zuckerman, 1951, Schultz, 1955, Dean and Wood, 1981, Dean and Wood, 1982, Luboga and Wood, 1990, Schaefer, 1999, Ahern, 2005 ). Despite some variation in the utility of different protocols for measuring foramen magnum position, these studies are all concordant in demonstrating that the foramen magnum and associated structures (e.g., occipital condyles) are more anteriorly positioned in humans than in any other extant primate species.

Which taxa have more anteriorly positioned foramina magna than pronograde?

Second, our data for strepsirrhines reveal that orthograde taxa (e.g., Propithecus, Galago, Lepilemur, and H. griseus) have more anteriorly positioned foramina magna than pronograde (e.g., Lemur and Varecia) or antipronograde taxa (e.g., Loris and Nycticebus ). This result suggests that the habitual assumption of orthograde trunk postures may lead to forward shifts in the relative position of the foramen magnum even in the absence of habitual bipedal locomotion. Indeed, when species mean ratios are compared across all of the primates in our sample ( Fig. 6 ), orthograde strepsirrhines have molar and temporal fossa ratios (but not palate ratios) that are broadly similar to those of non-human hominoids. This finding is interesting because all extant non-human hominoids are characterized by the assumption of more orthograde postures (particularly during arboreal climbing and suspension) compared with other catarrhine primates ( Cartmill and Milton, 1977 ). Nonetheless, humans (the only habitually bipedal living primate) have the lowest ratios in the primate sample and are outside the range of other groups. This observation suggests that habitual bipedalism in primates may be associated with a greater forward shift of the foramen magnum than that engendered by the assumption of orthograde postures alone.

Where are the bony landmarks located?

However, because the three bony landmarks used here are located on the splanchnocranium rather than the basicranium, our measures of relative basion position could arguably reflect variation in craniofacial morphology unrelated to foramen magnum position. In order to demonstrate that the bony landmarks chosen for this study are appropriate for investigating relative basion position, we also measured the distances between basion and the bicarotid and biporion chords for the hominoids in our sample. Though the use of the bicarotid and biporion chords as reference lines for assessing relative basion position in hominoids has received mixed support, basion–bicarotid distance and basion–biporion distance are the most widely accepted and reported measures of relative foramen magnum position. Following Ahern (2005), porion was estimated as the midpoint of a line connecting the anterior-most point and the posterior-most point on the external margin of the external acoustic meatus. The biporion line was then formed by connecting the left and right points on the specimen. Ahern (2005) defined a bicarotid chord by connecting the medial-most points on the carotid foramina. Here we chose to define a bicarotid chord by connecting the rostral-most points on carotid foramina because these two points are typically positioned in the same coronal plane 4 in our comparative sample. Once the bicarotid and biporion chords were defined, we measured basion–bicarotid distance and basion–biporion distance along a line drawn orthogonally from basion to each chord. Following Ahern (2005), these linear measurements were recorded as either positive or negative values depending on whether the biporion or bicarotid chords lie anterior or posterior to basion, respectively.

What is the foramen magnum?

The foramen magnum is a very important feature in bipedal mammals. One of the attributes of a biped's foramen magnum is a forward shift of the anterior border of the cerebellar tentorium; this is caused by the shortening of the cranial base. Studies on the foramen magnum position have shown a connection to the functional influences of both posture and locomotion. The forward shift of the foramen magnum is apparent in bipedal hominins →, including modern humans, Australopithecus africanus, and Paranthropus boisei. This common feature of bipedal hominins is the driving argument used by Michel Brunet that Sahelanthropus tchadensis was also bipedal, and may be the earliest known bipedal ape. The discovery of this feature has given scientists another form of identifying bipedal mammals.

Which part of the spinal cord is transmitted through the foramen magnum?

The spinal cord, an extension of the medulla oblongata, passes through the foramen magnum as it exits the cranial cavity. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the tectorial membranes and alar ligaments.

What is the hole in the skull called?

The hole indicated by an arrow is the foramen magnum. Occipital bone. Inner surface. The foramen magnum ( Latin: great hole) is a large oval opening ( foramen) in the occipital bone of the skull in humans and many other animals. It is one of the several oval or circular openings (foramina) in the base of the skull.

Which ligament divides the foramen magnum into an anterior compartment and a posterior compartment?

The alar ligament, which is attached on each side to the tubercle of occipital condyle, divides the foramen magnum into an anterior smaller compartment and a posterior larger compartment. Structures passing through anterior compartment (osseo-ligamentous compartment) include: Apical ligament and tip of dens.

Which hominins have forward shifts?

The forward shift of the foramen magnum is apparent in bipedal hominins →, including modern humans, Australopithecus africanus, and Paranthropus boisei. This common feature of bipedal hominins is the driving argument used by Michel Brunet that Sahelanthropus tchadensis was also bipedal, and may be the earliest known bipedal ape.

Where is the opisthion located on the foramen magnum?

On the occipital bone, the foramen magnum presents two midline cephalometric landmarks. The opisthion is the midpoint on the posterior margin of the foramen magnum. The basion is located at the midpoint on the anterior margin of the foramen magnum.

What muscle holds the head upright?

In humans, the foramen magnum is farther underneath the head than in the other great apes. Thus, in humans, the neck muscles (including the occipitofrontalis muscle) do not need to be as robust in order to hold the head upright.