Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaur Skeletal Anatomy

According to the authors1 their main sources for their book are references 2 and 3 below.

Skeletons of tetrapods, including dinosaurs, can be divided into a number of sections, from the point of view of topology, the skull, the vertebral column, that includes ribs and haemal arches, the pectoral girdle and forelimbs, and the pelvic girdle and the hindlimbs. Apart from the skull, or cranium, the remainder of the skeleton is referred to as the post cranium, the part of the skeleton behind the cranium.

The bones of the skull are comprised of pairs of bones, mirror images of each other lying on opposite sides of the axial plane, as well as unpaired bones lying along the midline of the skull. The main sense organs and brain are housed in the skull, and there may be teeth on the 2 bones of the ventral margin of the skull, the premaxilla and the maxilla, that together comprise the upper jaw. The teeth are absent from the premaxilla, from the maxilla, or from both, with a horny beak covering the mouth margins in some dinosaurs such as the ornithischians, titanosaurid sauropods, ornithomimid theropods, as well as living birds.

An opening, the antorbital fenestra, that is present in most dinosaurs, as well as in other archosaurian reptiles, is dorsal to the maxilla and the nasal opening is dorsal to the premaxilla. The antorbital opening had been reduced or even lost in some dinosaurs, such as the ornithischians, though the antorbital fenestra is wide and is associated with the other openings that were pneumatic, the maxillary and premaxillary openings, in others, especially the theropod dinosaurs. The paired premaxillae and the maxillae articulate dorsally with the paired nasals, and sometimes fused to form a single bone, surrounding the nasal passage.

The jugal bone surrounds the orbit (eye socket) on the ventral margin, the lacrimal rostrally (towards the snout), the postorbital caudally (towards the tail) and by the frontal on the dorsal margin, the frontal being a paired bone meeting along the midline of the skull that roofs part of the skull cavity. The frontals articulate with the nasals rostrally, with the parietals (that contribute to the braincase walls) caudally, and with the orbit-encasing bones laterally, such as the lacrimal and postorbital). The temporal fenestrae on the back of the skull housed the muscles of the jaw. The lateral temporal fenestrae on both sides of the skull, and the supratemporal fenestrae, on the dorsal skull surface. According to the author1 these paired fenestrae evolved in the common ancestor of the diapsids, a reptile group including lepidosasurs, such as lizards and snakes, and archosaurs, such as crocodilians, pterosaurs and dinosaurs, the latter including birds.

There is a left and right quadrate, a large bone allowing articulation with the lower jaw, on the posterior end of the skull of pterosaurs, the jaws may have teeth or a horny sheath. The dentary is the bone on the mandible on which teeth may be present, The left and right dentaries forming the symphysis that unites the 2 halves of the lower jaws. Where present, dinosaur teeth continued to grow throughout life, being replaced when required. The various dinosaur groups had teeth with different sizes and morphologies, depending on the dinosaur group. The teeth are conical among the Theropoda, but also compressed transversely, serrated and curved caudally. The teeth of sauropodomorphs are leaf-shaped, the margins of which were in some groups serrated, though there were some sauropods, that were derived forms, that had teeth like pencils. There was a variety of tooth types among the ornithischians, basal ornithischians having teeth that were spatulate and serrated, a prismatic type teeth without serrations were evolved  by lineages that were more derived.

The articulation of the dentary with the splenial is medial (towards the animal's midline), and caudally with 3 bones - with the surangular dorsally and with the angular (on the outer side of the mandible) and the prearticular (on the inner side of the jaw). The articular (the bone with a hinge joint articulating with the quadrate bone of the skull) meets these 3 bones caudally. A wide fossa, housing the adductor muscles that close the jaw, is bounded by the angular, surangular, prearticular and articular bones.

Several bones that are sutured tightly comprise the braincase, the braincase proper being beneath the outer skull's roof bones. The foramen magnum, at the back of the skull, is a large opening through which the spinal cord exits from the braincase. Unpaired bones bound the foramen magnum dorsally and ventrally, the supraoccipital and the basioccipital, respectively, and by the  paired occipitals laterally. The occipital condyle, a bulbous prominence formed by the basioccipital, that articulates with the vertebral column, i.e. the atlas and the axis.

The dinosaur vertebral column is comprised of 4 regions, the neck (cervical), the trunk (dorsal), the hip (sacral) and tail (caudal) vertebrae. Vertebrae of each of these sections have a characteristic morphology, and the number of vertebrae in each section varies according to the dinosaur group it belongs to. The cervical vertebrae are generally low, having a top to bottom dimension that is relatively small, and the length, front to back, that is usually elongate. The most robust vertebrae are those of the dorsal section. The sacrum of dinosaurs is comprised of 2 or more vertebrae, that are sometimes fused into a single structure. On either side the sacrum attaches to the left and right ilia, constituting the upper part of the pelvic girdle. Several vertebrae are included in the caudal series, with a tendency to reduce in size towards the tip of the tail.

The centrum, a large spool or cylindrical structure, forms the central structure of each vertebra, each centrum supports the neural arch dorsally. The neural canal, where the spinal cord is located, is bounded by the centrum and the neural arch on the dorsal side. On the centrum are cranial and caudal surfaces (the ends closest to the head and tail respectively) that articulate with contiguous centra. The articular surfaces may be flattened (amphyplatyan) to slightly concave (amphicoelous). The cranial surface is concave and the caudal end is convex (procoelous) in special cases, such as in the tail vertebrae of titanosaurid sauropods, and in the vertebrae of the necks of some derived theropods and sauropods, the reverse condition occurring, with the cranial surface being convex and the caudal end concave (episthocoelous). Pneumatic pores (pleurocoels) may perforate the sides of the centra. These foramina connect to the interior of the centra of the vertebra. The author1 has suggested they may have been passages for air sacs. Most saurischian dinosaurs have these perforations in their cervical vertebrae, and in some derived theropods and sauropods are also present in the vertebrae of the dorsal, sacral and caudal sections of the spinal column. In the centra with these pneumatic pores the internal region of the centra is hollow.

There is a dorsal neural spine projecting from the neural arch the provides surfaces for muscles to attach, as well as for tendons of muscles that pass over the vertebral column and ligament connections between successive neural spines. Pairs of finger-like projections (zygapophyses) are present both cranially and caudally. Articular surfaces that are oriented upwards and slightly inwards are present on the anterior zygapophyses (prezygapophyses) that support the posterior zygapophyses (postzygapophyses) on which the articular surfaces are oriented downwards and slightly outwards. Among the saurischians the vertebrae of the trunk section evolved additional surfaces for vertebral support, the hypantrum and the hyposphene. The hyposphene is a median wedge directed posteriorly that projects below the postzygapophyses and inserting into the hypantrum, a narrow space between the prezygapophyses.

There are prominences on the dorsal surfaces of the postzygapophyses on the cervical vertebrae of most dinosaurs, epipophyses, that provide attachment surfaces for the neck muscles.

Conspicuous apophyses project from the centrum and both sides of the neural arch of the vertebrae of the cervical and dorsal vertebral column. These apophyses are diapophyses when they extend laterally from the neural arch and parapophyses when they are present on the cranial margin of the cervical and dorsal vertebrae. The cervical and dorsal ribs, that are double headed, articulated with these paired apophyses, and the cervical and dorsal ribs are both split proximally into a dorsal process, the tubercle, that attaches to the diapophyses, and the capitulum, a ventral process attaching to the parapophysis.

The parapophysis migrates upwards and backwards from the centrum to the neural arch from one vertebrae to the next along the presacral series in archosaurs, including the dinosaurs, to finally be close to the apophysis, that may allow the fusion of the diapophysis with the parapophysis to form a single-headed structure. A ribcage (thorax) is formed around the vital organs from the ribs articulated to the dorsal vertebrae. The bellies of some dinosaurs also had gastralia ("belly ribs") that strengthened the ventral side of the animal, acting like a girdle holding the viscera. The author1 suggests they probably also contributed dynamically to the breathing.

There are transverse processes that project laterally from the neural arch of the caudal vertebrae that don't articulate distally with ribs (absent in the caudal series). Chevrons (haemal arches) are ventral to the tail (caudal vertebrae), that are Y-shaped in the cranial aspect, articulate below the point of connection between the 2 contiguous caudal centra.

There are ossified tendons passing along different parts of the vertebral column in some dinosaurs, as occurs in most of the ornithischians, that also have a net of ossified tendons that pass across the neural spines of the dorsal, sacral and neural spines, and also in the tail, across the haemal arches. Some titanosaurid sauropods evolved ossified tendons in the neck and some clades of theropods, such as abelisaurids and dromaeosaurids, evolved them in the tail.

Ligaments and muscles attached the pectoral (shoulder) girdle, to the anterior part of the ribcag. The pectoral girdle is comprised of an elongate scapula, that was somewhat strap-like, that articulated with the coracoid, a ventral element that is elliptical or of rectangular shape. The junction between the scapula and the coracoid along their caudal margin forms a deep concavity, the glenoid cavity (glenoid fossa) that articulates with the humerus, the proximal element of the forelimb. Paired clavicles ran over the cranial edges of both scapulae and coracoids, that had fused into a single bone, the furcula, in most predatory dinosaurs. The furcula is the "wish-bone" of birds. Along the midline of the body the coracoids of both sides connected to a paired sternum (breastbone) that was of varying shape among different dinosaur groups.

There are 3 main bones in the forelimbs of dinosaurs - humerus, ulna and radius. The proximal humerus joins the 2 forearm bones at the elbow. The humerus of most dinosaurs has a pronounced deltopectoral crest that is cranially directed that was the attachment for pectoral muscles from the chest. The ulna is larger than the radius and is more posterior than the radius, and there is a proximal prominence, the olecranon process, the attachment point for extensor muscles that strengthened the limb, and that is developed to varying degrees in different dinosaurs. The ulna and radius both articulate with the hand (manus). A series of small carpal (wrist) bones that articulate distally to metacarpals that were elongated, each being designated by a Roman numeral I to V. The most inner (medial) is I, and the most outer (lateral) is V. The numbering of the digits matches the numbering of the corresponding metacarpals. The phalanges are the individual bones of the digits. The unguals are the most distal of the phalanges, supporting horny claws or hooves.

In the primitive forms of dinosaurs they were bipedal reptiles that had forelimbs that were elongate, slender arms that were about half as long as the hindlimbs, and their hands had evolved to be more suited for manipulation than for walking/running. When some of the very large dinosaurs, such as sauropods, ankylosaurs, and stegosaurs, adapted to walk on 4 legs, the forelimb bones became more massive to support such great weights, though the number of digits on the hand remained almost unchanged. In the quadrupedal dinosaurs the ungual phalanges were either transformed into hooves or were lost completely. Among those retaining the bipedal posture, mainly the carnivorous theropods, the outer digit numbers were reduced, the unguals becoming proportionally large and trenchant. Dissimilar trends were manifested in the forelimbs of theropods, being strongly reduced in tyrannosaurids and abelisaurids, though the forelimb elongated in some derived theropods that were bird-like, such as deinonychosaurs, this was especially the case among flying birds.

On each side of the body the pelvic girdle is comprised of 3 bones, a dorsal bone (ilium), an anterovenrtral bone (pubis) and a posteoventral bone (ischium). There is a perforated depression, the acetabulum, that receives the femoral head., at the junction of the 3 pelvic bones. The sacral vertebrae connect to the left and right ilia. Expanded muscle attachment sites are provided by the ilium, ischium and pubis, mainly as the origins of several of the hindlimb retractors (that pull the hindlimbs back), as well as the muscles of the trunk and tail. The distal extremities of pubes and ischia are suggested by the author1 to possibly have been support points of the body when the animal was in resting posture.

A "triradiate" (propubic) pelvis was retained by most of the saurischians, was inherited from their archosaur ancestors, in which the pubis projects forwards and downwards. The ornithischians had a pelvis that pointed backwards (opisthopubic). Among the derived saurischians, such as the coelurosaurians therizinosaurids, mononychids, dromaeosaurids and birds, the pubis is retroverted, and independently developing a condition that superficially resembles that of ornithischian dinosaurs. 

The femur articulates with the pelvic acetabulum proximally through a femoral head that is medially offset, and joins distally to the tibia and fibula at the knee. There are different muscle attachment prominences on the femurs of dinosaurs, proximally the anterior trochanter, to which were attached fibres of protractor muscles originating on the ilium, and the 4th trochanter at caudal midshaft, for the attachment of retractor muscles originating on the ilium and tail. In the lower leg the tibia is the larger bone that is more medial, the fibula being the thinner bone that is more lateral. There is a rectangular-shaped cnemial crest that projects cranially on the proximal end of  the tibia, the fibula articulating with 2 tarsals (the bones of the ankle), respectively the astragalus and the calcaneum, that contact distally with a small number of distal tarsals that are disc-shaped. The tarsal bones, articulating with the metatarsals that are elongate (I-V in medial-to-lateral fashion, and distally the metatarsals articulate with their respective digits. The foot (pes) is the collective name of the digits, metatarsals and tarsals.


Sources & Further reading

  1. Novas, Fernando E., 2009, The Age of Dinosaurs in South America, Indiana University Press.
  2. Romer, A.S., 1956, Osteology of the Reptiles, Chicago University Press.
  3. Holtz, T.R. Jr.& Brett-Surman, M.K., 1997, The Osteology of Dinosaurs, in Farlow, J.O. & Brett-Surman, M.K.(Eds), The Complete Dinosaur, 78-91. Bloomington: Indiana University Press.


Images of the glenoid fossa


Author: M. H. Monroe
Last Updated 12/02/2012 



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