Shape of articular surface of crocodilian (Archosauria) elbow joints and its relevance to sauropsids

Abstract

The determination of area and shape of articular surfaces on the limb bones of extinct archosaurs is difficult because of postmortem decomposition of the fibrous tissue and articular cartilages that provide the complex three-dimensional joint surfaces in vivo. This study aims at describing the shape of the articular cartilages in the elbow joints of six crocodilian specimens; comparing its structure with that of four birds, three testudines, and five squamates; and comparing the shapes of the surfaces of the calcified and the articular cartilages in the elbow joints of an Alligator specimen. The shapes of the articular cartilages of crocodilian elbow joint are shown to resemble those of birds. The humerus possesses an olecranon fossa positioned approximately at the midportion of the distal epiphysis and bordering the margin of the extensor side of the articular surface. The ulna possesses a prominent intercotylar process at approximately the middle of its articular surface, and splits the surface into the radial and ulnar cotylae. This divides the articular cartilage into an articular surface on the flexor portion, and the olecranon on the extensor portion. The intercotylar process fits into the olecranon fossa to restrict elbow joint extension. Dinosaurs and pterosaurs, phylogenetically bracketed by Crocodylia and Aves (birds), may have possessed a similar olecranon fossa and intercotylar process on their articular cartilages. Although these shapes are rarely recognizable on the bones, their impressions on the surfaces of the calcified cartilages provide an important indication of the extensor margin of the articular surfaces. This, in turn, helps to determine the maximum angle of extension of the elbow joint in archosaurs.

What Lies Beneath: Sub-Articular Long Bone Shape Scaling in Eutherian Mammals and Saurischian Dinosaurs Suggests Different Locomotor Adaptations for Gigantism

“…In contrast, as shown by Bonnan et al [26] for extant archosaurs, thicker articular cartilage is associated with flatter, poorly-developed surfaces and a less convex shape. Here, too, this observation is not surprising to those who work with archosaur long bones and the many uncertainties that arise from the relative lack of articular data [25], [26], [50]. Thus, we conclude that subchondral bone which is relatively narrow in relation to the metaphysis and which displays pronounced geometry and convexity is strongly correlated with thin articular cartilage.…”

“…Certainly, although articular cartilage was significantly thicker than in mammals, there is no doubt that the articular surfaces themselves formed congruent joints. In fact, that the thick, cartilaginous articular cartilages articulated in congruent ways has been well-documented [25], [50]. However, the relatively thick articular cartilages of archosaurs must have imposed a different loading regime on the subchondral bone, and this is demonstrated by our results showing a very different pattern from mammals.…”

Elbow joint adductor moment arm as an indicator of forelimb posture in extinct quadrupedal tetrapods

“…Furthermore, determination of the typical scapular position and orientation on the ribcage still remains problematic or even overlooked [25]. Differences in the shapes of articular cartilage caps and calcified epiphyses can also be a problem, especially in fossil archosaurs whose cartilages are often expected to be thick [26][27][28][29][30].…”

Hox11 genes establish synovial joint organization and phylogenetic characteristics in developing mouse zeugopod skeletal elements

“…Thus, it will be of great interest to clarify whether there are regulatory connections and interplays among substandard limb rotation, zeugopod skeletal element shortening and joint restructuring that concurrently occur in our Hox11 mutants. RESEARCH REPORT Hox11 genes and limb synovial joint formation This schematic depicts the general organization and morphologies of elbow and knee joints in representative animal groups and species (Barnett and Lewis, 1958;Drapeau, 2004;Dye, 1987;Fujiwara et al, 2010;Haines, 1969;Soren and Waugh, 1994) compared with the joints in triple-Hox11-null mouse mutants. Most mammals have a conspicuous olecranon that encircles the distal end of the humerus, whereas the indicated groups of amphibians, reptiles and avians have a markedly smaller or even absent olecranon but have an ulnar patella (up, red).…”