Figure 1. Nyctosaurus pteroid and preaxial carpal
The Bone(s) At the Center of the Argument
The pteroid and the preaxial carpal are two unique wrist bones not seen in other tetrapods, only pterosaurs (and their kin). The pteroid is longer, pointier and tends to bend around the anterior distal radius (Figure 1). The preaxial carpal (always getting second billing) is short, stout and provided with a sesamoid in a dorsal pit (Bennett 2007). Dr. David Hone’s (2008) post “That troublesome pteroid” reported, “Thanks to a lack of direct pterosaurian ancestors in the fossil record [ed. note: Hone refuses to accept fenestrasaurs as pterosaur precursors] we are not sure where the pteroid comes from…” Workers have argued over the pteroid’s homology, articulation and orientation. Here are some answers based on the fossil record (Figure 1).
What is a Pteroid?
Early workers, like Goldfuss (1831) considered the pteroid digit #1 and the wing finger digit #5. Williston (1911) dismissed that hypothesis and wondered if the pteroid were 1) a misplaced first metacarpal or centrale; 2) a bone derived from sinew (tendon) or 3) an elongated sesamoid. Unwin et al. (1996) determined that the pteroid was a true bone, but could not determine its homology. Peters (2002) reported that the pteroid and preaxial carpal were homologs to centralia 1 and 2 of Romer (1956) because pterosaurs do not have centralia in their primitive positions. Peters (2009) found a tiny pteroid and preaxial carpal in Cosesaurus a phylogenetic precursor to pterosaurs. Here the pteroid and preaxial carpal are homologous with the centralia of Sphenodon, a taxon that shares a common ancestor with pterosaurs at the base of the Lepidosauria. Their migration of the centralia to the medial rim of the wrist is discussed below (Figure 2).
Figure 2. Click to enlarge. The origin of the pterosaur wing and the migration of the pteroid and preaxial carpal. A. Sphenodon. B. Huehuecuetzpalli. C. Cosesaurus. D. Sharovipteryx. E. Longisquama. F-H. The Milan specimen MPUM 6009, a basal pterosaur.
Early hypotheses placed the base of the pteroid in the dorsal pit of the preaxial carpal (Marsh 1882, Hankin and Waton 1914, Frey and Riess 1981, Padian 1984, Wilkinson et al. 2006). Bennett (2007) pointed out that the cup was preoccupied by a sesamoid and indicated that the pteroid articulated to the side of the preaxial carpal in a minor depression. Peters (2009) pointed out that Bennett’s (2007) own data demonstrated that the base of the pteroid actually articulated with the saddle-shaped joint on the proximal syncarpal (radiale) and only weakly contacted the proximal medial surface of the preaxial carpal.
Which Way Did the Pteroid Point?
Concerning its relative position some authors have postulated an anteriorly facing pteroid able to rotate to a variety of postions based on the suggested higher aerodynamic efficiency of a larger propatagium (Frey and Riess 1981; Wilkinson et al. 2006) and able to swivel while inserted within the preaxial cup. Both are wrong. All articulated pteroids face medially (Bennett 2007) on the anterior face of the radiale (proximal syncarpal, Peters 2009). The pteroid was a passive element, as are all carpals, moving only slightly as the propatagium was tightened or relaxed (Figure 6).
Figure 3. Click to Enlarge. Carpus of Sphenodon comparing the medial and lateral centralia to the pteroid (yellow) and preaxial carpal (blue) of pterosaurs.
How Did the Pteroid Come to Be?
A pterosaur precursor, Cosesaurus, also has a tiny pteroid, a preaxial carpal and its attendant sesamoid (Peters 2009), indicating a nonvolant origin for this unusual carpal configuration. Subsequent study has shown that Sharovipteryx and Longisquama also have a pteroid and preaxial carpal. Fenestrasaurs and all tritosaurs following Lacertulus and Huehuecuetzpalli lack centralia.
More primitive tetrapods, like Sphenodon, have centralia the same shapes as the pteroid and preaxial carpal in fenestrasaurs (Figure 3). The medial centrale in Sphenodon is long and pointed medially. The lateral centrale is short and stout. Neither are insertion or origin points for any muscles or tendons. They are only spacers, keeping the distal carpals apart from the proximal carpals. After migration in tritosaurs the distal carpals articulated with the proximal carpals. The intermedium and pisiform disappeared during the process, probably by fusion and reduction respectively.
Figure 4. Wing evolution in pterosaurs. Click to enlarge and animate.
Considering their original and ultimate orientation, the medial carpal likely emerged first, turned the corner, and was followed by the lateral carpal, which moved distally to the anterior face of the first distal carpal. Relieved of the intervening centralia, the distal carpals articulated with the proximal carpals as they do in pterosaurs. The migration time appears to be at the Lacertulus / Huehuecuetzpalli stage in which the entire carpus was poorly ossified.
Why Did The Pteroid and Preaxial Carpal Migrate?
At the Lacertulus / Huehuecuetzpalli stage the ancestors of pterosaurs were practicing bipedal locomotion (Carroll and Thompson 1982, Peters 2000) in a manner similar to living lizards capable of bipedal locomotion (Snyder 1954). Peters (2000) provided ample evidence for this. In addition, between Huehuecuetzpalli and Cosesaurus the coracoid fenestrations expanded leaving only the posterior rim as a strut. That meant the former sliding coracoid disc upon the interclavicle and clavicle was reduced to an immovable stem inserted into a cup, as in pterosaurs and birds. Since pterosaurs and birds flap their forelimbs, we can imagine that Cosesaurus, provided with the same type of coracoid, did so as well. Cosesaurus also developed precursor wing membranes in the form of trailing aktinofibrils (Peters 2009), which supports the flapping hypothesis. Since the pteroid was intimately associated with the propatagium in pterosaurs, it is safe to assume that some sort of minor propatagium was present in Cosesaurus, too (Figure 6). A propatagium is a passive restraint on elbow overextension and increases the wing area. Perhaps Cosesaurus gained stability and thrust from flapping while running with these new dermal extensions. Or perhaps these were secondary sexual traits, or both!
Evidence from Fenestrasaur Manus Tracks
The manus tracks pointed directly anteriorly in Rotodactylus and other lepidosaurs, while in pterosaurs they pointed laterally. By this evidence, Cosesaurus, a likely trackmaker of Rotodactylus and a pterosaur precursor, was still able to pronate its forarms. Pterosaurs could not. The loss of the intermedium and the straightening of the radius and ulna reduced the ability of the forearm to suppinate and pronate in pterosaurs (contra Bennett 2008). This continued straightening and lack of pronation was a product of bipedal locomotion together with flight restraints. For the same reasons neither bats nor birds can pronate or supinate their forearms.
Figure 5. Click to enlarge. Rotodactylus tracks. These digitigrade tracks match to Cosesaurus (Peters 2000). Digit 5 is posterior to the other four digits in both the manus and pes. The hands imprinted very close to the center line of the track.
How Did the Centralia Migrate?
The two centralia of Sphenodon, were neatly and inconspicuously tucked in at the same level with the rest of the carpals. When they migrated to a medial position on the wrist both were elevated above the traditional wrist contours, acting like twin violin bridges to pull the extensor tendon taut whenever the elbow was extended for flight. Originally the centralia were not insertion points for any muscles or tendons. Rather all the hand muscles and tendons passed over them and at right angles to their long axis (Figure 3). In pterosaurs the extensor tendons pass over the pteroid parallel to the long axis.
The migration of the centralia occurred when the carpus was poorly ossified. The centralia migrated little by little, generation after generation, while the manus was elevated in a bipedal configuration and while it was not being used for traditional quadrupedal lizard-like locomotion. In living lizards capable of bipedal locomotion, the hands do nothing. That was likely the case in the Late Permian/Early Triassic prior to the appearance of Cosesaurus. By then the hands were flapping, based on the morphology of the Cosesaurus coracoid (see above).
The Propatagium and Overextension of the Elbow
It is a common mistake in reconstructing pterosaur wings to overextend the elbow and extend the propatagium to the neck (Frey et al. 2006; Bennett 2007, 2008; Prondvai and Hone 2009). Bats and birds rarely extend the elbow more than 20 degrees beyond a right angle and pterosaurs would have been the same (Peters 2002). All available evidence indicates that the propatagium stretched between the pteroid and deltopectoral crest, not the neck.
Bennett’s (2008) Incorrect Reconstruction
Bennett (2008) proposed a strange reconstruction of pterosaur wing myology (musculature) by reversing the extensors and flexors. He reported, “…the range of motion of the metacarpophalangeal joint of digit IV migrated posteriorly so that flexor muscles spread the wing and extensor muscles folded it.” Bennett (2008, fig. 3) insisted that the pterosaur forearm was suppinated (radius over ulna in dorsal view) but illustrated the wing in the neutral position (radius anterior). His reconstruction failed to inserte and originate no muscles or tendosns on the dorsal or palmar sides of the distal carpus, and only two minor slips on the proximal carpus, contrary to the pattern of all other tetrapods.
Prondvai and Hone’s (2009) Big Hypothetical Tendon
Prondvai and Hone (2009) attempted to model a hypothetical biomechanical automatism to keep the wings open in flight in order to save energy, as in bats and birds. They proposed a propatagial ligament or ligamentous system which, as a result of the elbow extension, automatically performed and maintained the extension of the wing finger during flight and prohibited the hyperextension of the elbow. Unfortunately they overextended the elbow in their pterosaur and bat reconstructions, they avoided involving the pteroid in their ligament and they offered no evolutionary pathway to produce their hypothetical ligament. Instead, their imagined ligament ran like a rope through the middle of the propatagium. Nothing like this has ever been recovered in the soft tissue fossil record. Actually the biomechanical automatism was the entire propatagium, a likely homolog to the m. extensor digitorum longus of lizards and Sphenodon.
Figure 6. Sphenodon hand muscles
Extensor Muscles and Tendons
In lepidosaurus, such as Sphenodon (Haines 1939) and Polychrus (Moro and Abdala 2004), the m. extensor digitorum longus originates on the distal humerus, splits four ways distally and inserts on the proximolateral side of metacarpals 1-4, passing over both centralia (Figure 6). M. extensor digitorum longus does not extend to the base of the first phalanx as Bennett (2008) proposed and Prondvai and Hone (2009) supported in Sphenodon, but it may have done so in pterosaurs to facilitate automatic wing extension with elbow extension Figure 7). The short digital extensors of digits 1-3 originate on the intermedium (in Sphenodon) and insert on the distal metacarpals of digits 1-3 with a tendon extending to the ungual. They short extensors also pass over the lateral centrale. The short digital extensors of digit IV originate on the ulnare and distal ulna. The short digital extensor of digit 5 originates on the ulnare and pisiform.
Figure 7. Pterosaur wing flexed and extended, compared to Cosesaurus and demonstrating the hypothetical movement of the loosely articulated pteroid and preaxial carpal.
In fenestrasaurs, including pterosaurs, the intermedium fused to the radiale which served to straighten out the extensors of digits 1-3 in line with metacarpals 1-3. If the long extensor from the humerus did not send a slip to the wing finger, then the extensor digitorum brevis of digit 4 that originated on the distal ulna extended the wing finger. This tiny muscle seems unworthy due to the mass, moment and drag the giant wing finger would exert on it. Rather the expanded long extensor, now called the propatagium likely pulled the pteroid during elbow extension. The pteroid, in turn, pulled the wing finger open.
Sesamoids are defined as skeletal elements that develop within a continuous band of regular dense connective tissue (tendon or ligament) adjacent to an articulation or joint (Jerez et al. 2009). Lepidosaurs don’t have sesamoids on their centralia. They would have developed on the preaxial carpal at the point of stress.
According to the evidence, the pteroid and preaxial carpal were former centralia that had migrated to the medial wrist during early experiments with a bipedal configuration. The pteroid pointed medially and was a passive bone, unable to change its orientation except within the confines of a stretched or relaxed propatagium. During elbow extension, the propatagium pulled the pteroid medially, which extended the wing finger via a new set of tendons not seen in Sphenodon because its centralia were not involved in any tendons or muscles.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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