Sunday, November 30, 2014

Intramandibular joint

The unusual intra-mandibular joint described above is found only in herrerasaurids and theropods among dinosaurs. Dinosaurian outgroups (pterosaurs, crurotarsal archosaurs) also lack an intra-mandibular joint.


Page 21:
[Archaeopteryx] does not appear to have had an intramandibular joint
.....intramandibular articulation something that is actually absent in Archaeopteryx, but found in many of its theropod relatives.[2]

It would not tax the imagination to engender a long list of obstacles for the now dominant model of a theropod origin of birds, including....the sliding lower jaw joint [sliding intramandibular joint] of theropods (absent in birds)
"Kinetic dentary-surangular and splenial-angular articulations are also
present in theropods (e.g., Ceratosaurus, Fig. 10C; Carnotaurus, Bonparte et
al. 1990). In theropods, however, the articular surfaces of the
splenial-angular sliding joint are the reverse of that in Herrerasaurus and
Staurikosaurus; the tongue-shaped process of the splenial has a convex
dorsal articular surface that slides against a concave depression on the
angular. The dentary-splenial joint is also present in theropods, but the
posterodorsal process of the dentary is not elongated as in H.
ischigualastensis. The unusual intra-mandibular joint described above is
found only in herrerasaurids and theropods among dinosaurs. Dinosaurian
outgroups (pterosaurs, crurotarsal archosaurs) also lack an intra-mandibular
The traits uniting Theropoda seem to include:
An intramandibular joint between the dentary and post-dentary bones: this may have served as a shock absorber while feeding on live prey. (Herrerasaurs have a slightly different configuration of the intramandibular joint, and thus may be convergent.)
the analysis of Benton (2004) demonstrated that the only unequivocal synapomorphy diagnosing Theropoda is the presence of an intramandibular joint.
intramandibular joint absent
Although the majority of teleost fishes possess a fused lower jaw (or mandible), some lineages have acquired a secondary joint in the lower jaw, termed the intramandibular joint (IMJ). The IMJ is a new module that formed within the already exceptionally complex teleost head, and disarticulation of two bony elements of the mandible potentially creates a “double-jointed” jaw. The apparent independent acquisition of this new functional module in divergent lineages raises a suite of questions. 
Although the majority of teleosts possess a fused lower jaw as described above, some species have acquired an additional joint termed the intramandibular (“within the mandible”) joint, or the IMJ. The IMJ facilitates intramandibular bending, or movement that occurs between two individual bony components of the lower jaw; during this movement, the dentary bone rotates about its articulation with the angular-articular. The disarticulation of formerly fused bony elements of the mandible (via unknown developmental mechanisms) creates a “double-jointed” jaw in the species that possess this morphology. Thus, the IMJ is a new module that has formed within the already exceptionally complex teleost head. Remarkably, an intramandibular joint appears to have evolved independently multiple times (Fig. 2)—each time creating a “secondary” jaw joint and disarticulating two formerly fused elements of the lower jaw (Fig. 1).

Not all carnivores have such a joint. Only neotheropods appear have this, plus *Herrerasaurus* distinctly. Prosauropods and sauropods have flush dentary/postdentary margins or a semi-fixed herrerasaur pattern, and all ornithischians have reduced fenestra in the jaws that correlate with peg-in-notch and scarf joints between the two mandibular halves. The most dynamic predators today, such as falconiforms and cats, have very fixed,immobile jaws. Innovations in the joint were to increase gape and volume of the bite withing increasing skull size.
One factor which seems to favor bone-crushing as a significant behavior is the design of the tyrannosauroid jaw. Older reconstruction of tyrannosaurs usually incorporated the typical theropod jaw, which includes an intramandibular joint. This joint connects the anterior dentary, splenial and (if present) supradentary with the posterior surangular, angular, coronoid, prearticular and articular. Since this connection is hinged, the lower jaw bends outward in the middle when it is stressed -- as, for example, when the teeth hit something rather hard. Accordingly, most bone would not be crushed. Rather the teeth would slide over it as the jaw deformed, causing the bone to be swallowed whole or rejected.
This arrangement is certainly operative in carnosaurs, and perhaps even basal tyrannosauroids. However, Hurum & Currie (2000) have shown that tyrannosaurinids block the joint. The supradentary overgrows the joint and fuses with the coronoid. This connection is reinforced by a long process of the prearticular which articulates with both the coronoid and the splenial. An anterior process of the angular also bridges the gap ventrally. In addition, the supradentary sends ridges between the teeth, further immobilizing the lower tooth row.


  1. For future reference:
    Page 135
    Symmetric feathers twist.

    In the mid-length, the mandible is divided by the intramandibular joint. The articular, angular, surangular and coronoid are incorporated into the caudal structural unit, while the splenial and dentary form the rostral one.

    In theropods and Archaeopteryx (both mandibles of JM2257) the long, slender but strong anterior process of the angular lies in a long, subhorizontal groove of the splenial. The action of this joint has not been analyzed in detail, but vertical rotation of the dentary-splenial unit upon the posterior mandible appears to be favored.