Megalancosaurus was first considered to be arboreal. However Megalancosaurus was also cited as aquatic and it has also been suggested that this reptile was not only a good climber with arboreal habits, but also a skilled glider.
Any hypothesis on the functional morphology of Megalancosaurus, should take into account the huge amount of specialized features present in its skeleton and put them together in a consistent model. In Megalancosaurus there are: 1) a triangular skull, posteriorly inflated with a narrow, "beak-like" snout, 2) a long neck with high vertical mobility ; 3) a "notarium-like" structure, with this term it is indicated here the presence of a sort of supraneural plate formed by expansion and fusion of the neural spines of anterior dorsal vertebrae , which is superficially similar to the notarium of some pterodactiloyd pterosaurs; 4) a barrel shaped trunk, that is very rigid owing to the position of the the zygapophyses (close to the midline) and to the fusion of the ribs with the walls of the neural arches, 5) a high pectoral girdle with clavicles, possibly also with sternal plates, along with a very high and extremely narrow scapula which is vertically oriented, and a glenoid facing laterally and posteriorly; 6) a pelvic girdle with a very high anteriorly inclined iliac blade; 7) slender and rather long limbs; 8) carpus and tarsus allowing a high degree of rotation of both the manus and pes allowing a high degree of rotation of both the manus and pes 9) manus with opposable digits and stout, long claws, 10) pes with long toes ending in very long narrow and sharp claws and at least in one morph an opposable clawless first toe. 11) tail long, stiff in the middle portion (due to the long prezigapophyses, lying close to the midline and laterally enveloping the preceding vertebra), but with good possibilities of flexion in the vertical plane at its base and forming a robust "hook" at the end; in addition this latter bears, a pointed claw-like terminal spine. 12) tail laterally compressed and leaf- like in appearance.
The long narrow snout and the small triangular teeth are useful for grasping and keeping insects. The very large eyes and inflated parietal region may be indicative of good, vision, useful for locomotion and precise prey location among tree twigs and possibly good limb coordination.
The morphology of the cervical vertebrae is similar to that of some pterosaurs (e. g. the buffer-like zygapophyses are present also in some rhamphorhyncoid pterosaur) and it may be suggested that the neck of Megalancosaurus was very flexible only on the vertical plane, but torsion was eliminated by the "hypapophyses" and lateral mobility was limited by the buffer-like zygapophyses. The presence of very high neural spines in the anterior dorsal vertebrae suggest that robust muscles or tendons may have been originated from these structure permitting the neck to be held upright. The hypapophyses, in addition, allowed the insertion of a well developed longus colli, for the extension of the neck. All these features permitted the neck of Megalancosaurus to be both retracted and suddenly projected forward, as in some projectile feeders. This pattern is at least superficially similar to a miniaturized version of some long necked pterodactyloid pterosaurs.
The height of the neural spines all along the dorsal region and the rigidity of the rib cage are consistent with the ability to raise the neck and the trunk off the substrate by development of a robust transversospinalis system. Further support to this hypothesis comes from the shape of the ilium.
Anterior dorsal region, shoulder girdle and humerus of MPUM 6008
The notarium like supraneural plate in MPUM 6008 (above) and in the holotype (below).
The supraneural plate formed by the anterior dorsal vertebrae, together with the problematic supraneural bones is the most difficult feature to be explained. In pterodactyloid pterosaurs the supraneural plate served to articulate the free end of the scapula and is also the area for the insertion of a very powerful latissimus dorsi, a retractor and elevator of the humerus; this muscle plays an important role in raising the humerus backwards above the glenoid level.
The scapula from specimen MPUM 6008 (left) and holotype (right).
The morphology of the scapula is quite similar to that of chameleons. However, the discovery of clavicles, and possibly of sternal plates, complicates this interpretation. Clavicles would render the pectoral girdle more solid, and prevent to some extent a lateral swing during walking, a feature that allows greater reach to the forelimb of chameleons. Thus the clavicles of Megalancosaurus may have limited the anterior extension of the fore limb, and at the same time provided more rigidity to the girdle. If the scapula was connected to the supraneural plate as in pterosaurs, the shoulder girdle was indeed rigid. The pattern of the carpus and of the tarsus suggests good mobility, with wide possiblities of flexion and rotation at the wrist and ankle level for better grasping abilities. It may be hypothesized that the stout structure of the shoulder region may have allowed good brachiation and quick movements of the fore limbs. Thus, without excluding their use for locomotion on narrow branches, the anterior limbs may have been also useful for seizing or holding firmly the prey. The hands of Megalancosaurus with opposable digits may have been capable of catching and holding prey like insects, grasping to the cuticle with the very stout and sharp claws.
From left to right: The hands of specimen CCSR , holotype and MPUM 6008 .
The lateral expansion on the sides of the ungual phalanges of the pes indicate that very long cheratinous sheaths were present in life. This occurs frequently in climbing vertebrates. The modified first toe which is present in some Megalancosaurus specimens, (and in Dolabrosaurus) may represent an useful device for single extremity grasping on slender supports, but, however, if the modified first toe was a sex-related feature, it may have served perhaps to hold the partner during mating on unstable supports like narrow twigs.
The shift of the insertion of the haemal spines in the posterior portion of the tail is probably related to a prehensile tail and to the presence of the terminal spine. It is worth noting that the same feature also occurs in Dolabrosaurus (which suggests that Dolabrosaurus may also have been arboreal in habit).
The hooked spine of a specimen from Friuli
The terminal hooked spine, is rather an odd structure that can be explained as a device for a particular kind of prehension. Chameleons grasp twigs and branches by curling the narrow, tail circular which is circular in section. The deep, leaf-shaped tail of Megalancosaurus was less suited for this task, but the terminal hook may have been a functional substitute. The tight articulation of the spine with the preceding vertebra would have kept the spine in a nearly fixed position: it pointed anteriorly and lay almost parallel to the axis of the tail both in Drepanosaurus and in Megalancosaurus forming a stiff hook. In such a position it may have been useful not only to grasp on twigs, but also to "lock" the animal by clinging in crevices or in cracks of the substrate, counteracting gravity. The same device may have also been useful when Megalancosaurus was hanging in an head down position.
In summary, the morphology of the end of the tail and posterior limbs of all known drepanosaurids follows the pattern of climbing vertebrates with prehensile tails and grasping hind feet that may be used together forming a stable three point support base for stance or feeding, also hanging head down. It has to be noted that the first caudal vertebrae bear narrow neural spines that are inclined posteriorly, allowing to rise the tail at its base to some extent. In Drepanosaurus the neural spines of the posterior sacral and proximal caudal vertebrae were low, perhaps for the same reason.
It is suggested here that Megalancosaurus may have hunted by ambush among foliage, moving slowly or perhaps, remaining still in a tripod stance, with the neck and anterior limbs in a retracted position. When the prey was at a suitable distance, it was seized with a sudden extension of the limbs and of the long neck. The victim was then held firmly with the grasping hands. This would have rendered Megalancosaurus hunting mode more similar to that of a praying mantis rather than to that of a chameleon.
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