Saturday, November 20, 2010

Megalancosaurus is not a theropod or a bird ancestor

 Another quickie from the ex-theropod files. 

Megalancosaurus Calzavara, Muscio and Wild, 1980
M. preonensis Calzavara, Muscio and Wild, 1980
Middle Norian, Late Triassic
Dolomia di Forni Formation, Italy

Holotype- (MFSN 1769) skull (30 mm), mandibles (18 mm), hyoid, several cervical vertebrae (third 7 mm, sixth 9 mm), second dorsal neural spine, fused third and fourth dorsal neural spines, two dorsal ribs, dorsal rib fragments, scapula (23 mm), coracoid fragment, humerus (22 mm), radius (15 mm), ulna (15 mm), intermedium, ulnare, centrale, distal carpal I, distal carpal II, distal carpal III, metacarpal I (1.5 mm), phalanx I-1 (6 mm), partial manual ungual I, metacarpal II (3 mm), incomplete phalanx II-1 (6 mm), metacarpal III (3.5 mm), partial phalanx III-1 (6 mm), manual ungual III (4 mm), metacarpal IV (3 mm), partial phalanx IV-1 (3 mm), phalanx IV-2 (5 mm), manual ungual IV (4.5 mm), metacarpal V (2 mm), partial phalanx V-1 (3 mm), phalanx V-2 (4 mm), manual ungual V (4.5 mm)
Referred- (MFSN 1801) caudal vertebrae 8-38 fused to chevrons (Pinna, 1987)
(MFSN 18443a) caudal vertebrae 13-38 (Renesto, 2000)

Middle Norian, Late Triassic
Zorzino Limestone Formation, Italy

(MBSN 26; paratype of Drepanosaurus unguicaudatus) cervical vertebrae (third 4.5 mm, sixth 6 mm), anterior dorsal vertebrae (second 4 mm), anterior dorsal ribs, posterior dorsal vertebrae fused to dorsal ribs (eighteenth 3 mm), supraneural element, caudal vertebrae fused with chevrons (tenth 5.5 mm), scapulae (21 mm), humerus (18.5 mm), radius, ulna, pelvis, femur (20 mm), tibia (11.5 mm), fibula, pes (Pinna, 1980)
(MPUM 6008; = P 11 24) cervical vertebrae (third 7.5 mm, fifth 9.5 mm, sixth 10 mm), anterior dorsal vertebrae (second 6 mm), anterior dorsal ribs, posterior dorsal vertebrae fused to dorsal ribs, supraneural element, sacral vertebrae, caudal neural spines, scapula (23 mm), coracoids, humerus (22.5 mm), radius (14 mm), ulna (14 mm), proximal carpal, four distal carpals, phalanx I-1 (6 mm), manual ungual I (4 mm), metacarpal II (3 mm), phalanx II-1 (6 mm), manual ungual II (4 mm), metacarpal III (3 mm), phalanx III-1 (6 mm), manual ungual III (4 mm), metacarpal IV (2.5 mm), phalanx IV-1 (3 mm), phalanx IV-2 (5 mm), phalanx V-1 (2.5 mm), phalanx V-2 (4 mm), manual ungual V (3.5 mm), three manual phalanges, two manual unguals, partial pelvis (Renesto, 1994)
(MPUM 8437; = CCSR 63115) posterior skull, incomplete mandible, eight cervical vertebrae (fifth 7 mm, sixth 7.5 mm), five anterior dorsal vertebrae, anterior dorsal ribs, eighteen posterior dorsal vertebrae fused to dorsal ribs, supraneural element, three sacral vertebrae, thirty-nine caudal vertebrae fused to chevrons, partial scapula, partial coracoid, clavicle, distal humerus (~21 mm), radius (11 mm), ulna (12 mm), intermedium, ulnare, two centrales, distal carpal I, distal carpal II, distal carpal III, distal carpal IV, distal carpal V, metacarpal I, phalanx I-1, partial manual ungual I, metacarpal II, incomplete phalanx II-1, incomplete manual ungual II, metacarpal III (4 mm), phalanx III-1 (5 mm), manual ungual III, metacarpal IV, phalanx IV-1, phalanx IV-2, manual ungual IV, metacarpal V, phalanx V-1, incomplete phalanx V-2, manual ungual V, partial pelvis, femora (27 mm), tibiae (17.5 mm), fibulae (16 mm), astragali, calcanea, centrale, distal tarsal I, distal tarsal II, distal tarsal III, distal tarsal IV, metatarsal I, phalanx I-1, pedal ungual I, metatarsal II, phalanx II-1, phalanx II-2, pedal ungual II, metatarsal III (4 mm), phalanx III-1 (3 mm), phalanx III-2, pedal ungual III, metatarsal IV, phalanx IV-1, phalanx IV-2, phalanx V-1, phalanx V-2, pedal ungual (Renesto, 2000)

Late Triassic?

(MCSNB 7833) (Senter, 2004)

Comments- Megalancosaurus preonensis was discovered in 1980 and originally assigned to Pseudosuchia sensu Huene (Calzavara et al., 1980). Although Carroll (1988) also placed it in Thecodontia (in the traditional paraphyletic sense), generally only those workers who reject cladistics and a dinosaurian origin for birds have continued to call Megalancosaurus an archosaur (e.g. Feduccia and Wild, 1993; Feduccia, 1996). This is almost exclusively based on its supposed antorbital fenestra (considered near certainly absent by Renesto and Dalla Vecchia, 2005), and presumably the need to have birds derive from archosaurs. Megalancosaurus is currently placed in the larger clade Simiosauria, which has a highly uncertain placement among eosuchians. Various analyses place them outside Neodiapsida, sister to Euryapsida, in Lepidosauromorpha, as non-thecodont archosauromorphs or as 'protorosaurs', with the latter three possibilities sometimes including a close relationship with pterosaurs. Determining their relationships will require a larger diapsid phylogenetic analysis than those currently published.

Megalancosaurus a theropod? Olshevsky (1991) believed Megalancosaurus to be a basal theropod (or in his taxonomy, a basitheropod theropodomorph), but this was based only on the holotype. Of the theropodomorph characters he lists, carnivorous dentition is primitive for gnathostomes, while new specimens show Megalancosaurus lacks erect limbs and a reduced calcaneum. Of Olshevsky's basitheropod characters, an antorbital fenestra is primitive for archosauriforms and probably lacking in Megalancosaurus, "generally avian appearence of the skull" is vague and unlike basal theropods, relatively large forelimbs are primitive for tetrapods and unlike basal theropods, "clavicles, fused clavicles, or primitive furcula" covers every possibility and Megalancosaurus' are unfused which is primitive for tetrapods, and pentadactyl manus and pes are plesiomorphic for tetrapods and not found in basal theropods. The tarsus is not even incipiently mesotarsal and as noted above the calcaneum is not reduced. Megalancosaurus does share the presence of at least three sacral vertebrae with dinosaurs, but this is present in pterosaurs and some other taxa as well. Furthermore, the more basal Vallesaurus and Drepanosaurus only have two sacrals. While Megalancosaurus and theropods both have manus capable of grasping, in theropods digit I is angled towards II and III due to an asymmetrical metacarpal I articulation and twisted phalanx I-1, whereas in Megalancosaurus half the digits oppose the other half due merely to a lack of articulation between the metacarpals. This suggests the grasping abilities are convergent.  

Megalancosaurus lacks numerous characters expected in a basal theropod, such as subnarial fenestra, external mandibular fenestra, thecodont dentition, more than eight cervicals, cervical epipophyses, vertebral laminae, dicephalous dorsal ribs, reduced manual digits IV and V, perforated acetabulum, dorsally angled preacetabular process, elongate postacetabular process, elongate pubis and ischium, inturned femoral head, anterior and fourth trochanter, mesotarsal ankle, reduced calcaneum, centrale absent, less than three distal tarsals, and reduced pedal digit V. These make it virtually impossible to assign Megalancosaurus to Dinosauria, let alone Theropoda.

Megalancosaurus a bird ancestor? Feduccia and Wild (1993) first suggested Megalancosaurus was more closely related to birds than theropods were, though the absence of other taxa in their cladogram leaves one uncertain exactly where in Archosauria they place the the Megalancosaurus+bird clade. The expanded braincase is also present in pterosaurs and coelurosaurs. Large orbits are found in those two groups as well, and many other small tetrapods. A pointed snout is also present in many maniraptoriforms, pterosaurs, most 'protorosaurs', choristoderes, thalattosaurs, and some basal lepidosauromorphs. The "large, oval preorbital fenestra" is actually the external naris. The reduced premaxillary dentition is not present in basal birds, though the authors state it "may be important, because when modern birds loose their teeth they loose the maxilla that houses them, and in the Cretaceous toothed birds, the teeth are borne only on the maxilla." Even ignoring the fact Aves does retain a maxilla, and subsequently discovered birds like omnivoropterygids and longipterygids have toothed premaxillae and toothless maxillae, mere propensity for a group to exhibit a character is not a synapomorphy. The dentary is said to be birdlike, but this is too vague to evaluate. The foramen magnum is claimed to be posteroventrally oriented, but this is based solely on the angle of the quadrate and ventral squamosal process. The latter is also true in Icarosaurus, the basal squamate Tamaulipasaurus, the basal choristodere Lazarussuchus, Cosesaurus and pterosaurs. Feduccia and Wild state "the six to seven elongate cervical vertebrae create a highly movable birdlike neck." The high mobility is caused by heterocoelous centra, which are similar to ornithuromorphs but not basal birds. Elongate cervical centra are present in coelurosaurs, 'protorosaurs' and other taxa, while birds have at least nine cervicals. Though the authors claim the "anterior limbs, without manus, are proportionately similar to those of modern birds and Archaeopteryx; and dissimilar to theropods," the radiohumeral ratio of 68% is shorter than most maniraptorans (even taxa known in 1993, e.g. Oviraptor 87%, Deinonychus 76%) and Archaeopteryx (84-96%), while modern birds usually have ratios of more than 100%. The large manual ungual flexor tubercles are said to be birdlike, but these are absent in the more basal Hypuronector, and also found in most theropods and pterosaurs. Contra Feduccia and Wild, the forelimbs minus manus are not longer than the hindlimbs minus pes, being 72% as long instead. Finally, the straplike scapula is indeed superficially like Aves in being extrenmely slender, bowed and having a tapered distal end. Yet basal birds lack these features, and only have scapulae as slender as pterosaurs and most theropods.

Feduccia later (1996) refers to Megalancosaurus' "tiny isodont teeth set in sockets", but they are actually subthecodont as in most basal diapsids, while isodonty is also plesiomorphic.

After the mid-1990s, Feduccia and other Birds Are Not Dinosaurs supporters seem to have reduced their emphasis on Megalancosaurus' relationship with birds. Geist and Feduccia (2000) repeat the 1993 observations, but then state "though probably not the avian ancestor, Megalancosaurus represents a chronologically and biophysically plausible model for a gliding stage through which birds must have passed." Martin (2004) incorrectly stated Megalancosaurus has a furcula, but did not explicitly link it to bird origins.

Besides those characters noted above that exclude Megalancosaurus from Theropoda, it is less similar to basal birds than even basal coelurosaurs are in many other ways. These include the absence of pleurocoels, less than five sacral vertebrae, absence of a transition point in the tail, unfused clavicles, small distal carpal I, metacarpal III longer than II, presence of manual digits IV and V, absent pubic boot, absent obturator process, obturator foramen in pubis, astragalus without tall ascending process, robust metatarsus, metatarsal I reaching tarsus, and phalanx V-1 present.

References- Calzavara, Muscio and Wild, 1980. Megalancosaurus preonensis, n. g., n. sp., a new reptile from the Norian of Friuli. Gortania. 2, 49-63.

Pinna, 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di lepidosauro del Trias alpino. Atti della Società Italiana di Scienze Naturali - Museo civico di Storia Naturale di Milano. 121, 181-192.

Pinna, 1987. Un nuovo esemplare giovanile di Drepanosaurus unguicaudatus del Norico di Val Preone (Udine) [A new juvenile specimen of Drepanosaurus unguicaudatus from the Norian of the Preone Valley, Udine]. Atti della Società Italiana di Scienze Naturali - Museo civico di Storia Naturale di Milano. 128, 80-84.

Olshevsky, 1991. A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Mesozoic Meanderings. 2, 196 pp.

Feduccia and Wild, 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Naturwissenschaften. 80, 564-566.

Renesto, 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of northern Italy. Journal of Vertebrate Paleontology. 14(1), 38-52.

Feduccia, 1996. The Origin and Evolution of Birds. Yale University Press. 420 pp. 

Ruben, 1998. Gliding adaptations in the Triassic archosaur Megalancosaurus. Journal of Vertebrate Paleontology. 18(3), 73A.

Geist and Feduccia, 2000. Gravity-defying behaviors: Identifying models for protoaves. American Zoologist. 40, 664-675.

Renesto, 2000. Bird-like head on a chameleon body: New specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of northern Italy. Rivista Italiana di Paleontologia e Stratigrafia. 106(2), 157-180.

Martin, 2004. A basal archosaurian origin for birds. Acta Zoologica Sinica. 50(6), 978-990.

Senter, 2004. Phylogeny of the Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology. 2, 257-268.

Renesto and Dalla Vecchia, 2005. The skull and lower jaw of the holotype of Megalancosaurus preonensis (Diapsida, Drepanosauridae) from the Upper Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia. 111(2), 247-257.

Renesto, Spielmann, Lucas and Spagnoli, 2010. The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepanosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha). New Mexico Museum of Natural History and Science Bulletin. 46, 1-81.

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