Saturday, July 24, 2010

Longisquama is not a dinosaur

Longisquamidae Sharov, 1970
Longisquama Sharov, 1970
L. insignis Sharov, 1970
Ladinian, Middle Triassic
Madygen Formation, Kyrgyzstan

Holotype- (PIN 2584/4) incomplete skull (~23 mm), incomplete mandibles, eight cervical vertebrae, several anterior dorsal vertebrae, dorsal ribs, scapula (12 mm), coracoid (5 mm), interclavicle? (9 mm), furcula, sternum?, humerus (13 mm), radii, ulnae (11 mm), radiale, intermedium, ulnare, pisiform, four distal carpals, metacarpals I, phalanx I-1, manual ungual I, metacarpals II (one proximal), metacarpals III (one proximal), proximal phalanx III-1, metacarpals IV (one proximal), phalanx IV-1, phalanx IV-2, phalanx IV-3, phalanx IV-4, manual ungual IV, metacarpals V, phalanx V-1, phalanx V-2, phalanx V-3, manual ungual V, scales, parafeathers
Paratypes- (PIN 2584/5) partial parafeather
(PIN 2584/6) two partial parafeathers
(PIN 2584/7) partial parafeather
(PIN 2584/9) six distal parafeathers
Referred- (FG 596/V/1) parafeather (Voigt et al., 2009)
(FG 596/V/2) parafeather (Voigt et al., 2009)
(FG 596/V/3) parafeather (Voigt et al., 2009)

Comments- Longisquama was originally described as a pseudosuchian, which at the time was used for all basal archosauriforms that were not parasuchians. Surprisingly few authors have addressed its relationships since, traditionally assuming it to be an archosaur(-iform) because of the supposed antorbital and mandibular fenestrae. More recently, most reviewers (e.g. Reisz and Sues, 2000) have been even more cautious and refer to it merely as a diapsid. The matter is especially difficult to resolve as the holotype only received a short original description with poor illustrations and has not been osteologically described since, with the exception of Peters' (2000) seemingly overimaginative attempt. Several characters have proven controversial and will be discussed first. Though originally described as having an antorbital fenestra, Senter (2003) found this was caused by breaks which were not present in the main slab, a conclusion also reached by Prum (2001). Peters (2000) and Martin (2004) both disagree however, believing it to have maxillary and promaxillary fenestrae as well. Similarly, an external mandibular fenestra was originally described, though all authors including Peters, Martin and Senter now agree this was due to damage. The mode of tooth implantation was described as acrodont by Sharov, though Martin believes it is thecodont. He provided no evidence for this in 2004, and even qualified the character with a question mark. In a later 2008 paper, Martin states the mandible had become split between slabs, and what Sharov interpreted as tooth crowns were actually entire teeth with expanded bases. Yet supposing the mandible with preserved teeth is being viewed laterally, the teeth would more probably be pleurodont since no lingual wall is apparent. Certainly the roots would be too short for thecodont teeth in any case, though they might be subthecodont if socketed. The posterodorsal skull is preserved expanding significantly past the orbit, described as two tubercles by Sharov, and interpreted as a crest by Peters and Senter. Martin (2008) believes it is merely part of the skull roof that has become disarticulated. Either interpretation seems possible. The clavicles were described by Sharov as fused ("concrescent") though a suture was illustrated. Senter and Martin agree they are fused to form a furcula, Peters thinks they merely overlap each other, and Unwin and Benton (2001) believe they are unfused (incorrectly saying this was Sharov's opinion). Senter (2003, 2004) and Peters (2000) have been the only authors to include the taxon in a phylogenetic analysis.

Longisquama a theropod? Olshevsky (1991) believed Longisquama to be a basal theropod (or in his taxonomy, a basitheropod theropodomorph), but of his noted characters for that group, it only has "generally avian appearence of the skull" (vague and unlike basal theropods), carnivorous dentition (plesiomorphic for gnathostomes), furcula (somewhat uncertain), relatively large forelimbs with pentadactyl manus (plesiomorphic for tetrapods and not found in basal theropods), and "featherlike scales" (which is problematic, as parafeathers do not seem to be scales or necessarily homologous with feathers, and scales are not homologous with feathers in any case). While the presence of a furcula would be theropod-like, Peters' (perhaps incorrect) interpretation would have it posteroventrally concave and fused along its length to the sternum, quite unlike the condition in theropods. Additional characters more plesiomorphic than dinosaurs include the absent external mandibular fenestra, tooth implantation (whether acrodont, pleurodont or subthecodont), interclavicle (if correctly identified), short deltopectoral crest, five phalanges on manual digit IV and four phalanges on digit V. It is thus near certainly not a dinosaur, theropod or otherwise.

Longisquama a bird ancestor? Another connection to dinosaurs has been the claim that Longisquama is related to the ancestor of birds. This originated with Sharov (1970), who believed the furcula and elongate forelimb scales were birdlike, but has more recently been popular among those arguing birds are not dinosaurs (beginning with Jones et al., 2000). Besides the furcula and forearm scales (which cannot be homologous with secondary feathers based on developmental data), Martin (2004) listed several other characters as being similar to birds- subdivided antorbital fenestra (absent as noted above), pointed snout (also in Coelurosauravus, simiosaurs, pterosaurs, most coelurosaurs, and numerous other taxa), "expanded cranium" (uncertain even if the supposed crest is really a displaced skull roof, as its three dimensional placement is unknown, as is endocranial size; also in Megalancosaurus, pterosaurs and coelurosaurs), elongate postorbital (vague and actually reduced in most birds, with even basal forms like Archaeopteryx having a shorter ventral ramus that ironically resembles Coelurosauravus more), absent mandibular fenestra (variable in basal birds and true of almost all non-archosauriforms), teeth with expanded roots (unverified as noted above), neck attaches low to skull (only true if the parietal crest is taken as part of the skull, and not actually true in basal birds like Archaeopteryx), strap-like scapula (also present in neotheropods, simiosaurs, pterosaurs and to a lesser extent in Coelurosauravus), elongate manus (also in pterosaurs, coelurosaurs and many other taxa), elongate penultimate manual phalanges (untrue in digit I, and only homologous in Martin's view in digit IV; also present in Coelurosauravus, simiosaurs, pterosaurs and theropods), and feathers.

Longisquama parafeather (after Voigt et al., 2009). al- anterior lobe, ml- middle lobe, pl- posterior lobe, ru- transverse ridges.

The supposed feather homologs of Longisquama have generated the most research since Jones et al. first redescribed them, and have since been more accurately described by Voigt et al. (2009). Jones et al. described a number of similarities to feathers, and it seems fitting to use Feduccia's (2002) term 'parafeather' for the structures. Though Haubold and Buffetaut (1987) proposed the parafeathers were paired and could be horizontally extended as gliding surfaces (a claim followed by Martin), there is no evidence of this and Voigt et al. noted any such 'thoracic wing' would be compromised by having the aerodynamic surfaces so distally placed. The cylindrical, tapered base is similar to follicular structures like feathers, though Voigt et al. noted some scales such as those on iguanid dorsal frills have this characteristic as well. However, the supposed transverse partitions homologized to avian pulp caps by Jones et al. (and claimed to be pedal phalanges by Peters, 2006) are actually transverse ridges on both sides of the parafeather's middle lobe. The supposed calamus walls surrounding them are the anterior and posterior lobes, which lack ridges basally (Voigt et al., 2009). While parafeathers look roughly feather-like distally in having a central shaft and surrounding vane, the actual structure is quite different. Instead of a hollow rachis and separate barbs to form the vane, Longisquama has a pair of membranes which join at their edges (Reisz and Sues, 2000) and enclose two longitudinal lobes distally, as the posterior lobe tapers out before the vane-like expansion. The shaft analog is a continuation of the boundary between the anterior and middle lobes (so is not even continuous with the basal 'calamus' as identified by Jones et al.), while the supposed barbs never separate even at the parafeather's tip and often merge (blamed on taphonomy by Jones et al.). Instead, the 'barbs' are transverse ridges in the continuous membrane. There seems to be an outer sheath on the base of each parafeather, which was homologized by Jones et al. with the sheaths on avian feathers. Resemblences to feathers thus seem limited to the cylindrical and tapered base and basal sheath. While parafeathers may be homologous to feathers at the level of the follicle, they are no more similar than the stage 1 feathers of Tianyulong and basal coelurosaurs, the quills of Psittacosaurus or the pycnofibres of pterosaurs.

Longisquama does not share any characters with birds not found in basal coelurosaurs, and can be excluded from Tetanurae based on numerous characters such as maxillary teeth extending posteriorly under the orbit, having only eight cervical vertebrae, lacking an enlarged distal carpal I+II, having phalanges on manual digit IV and having digit V, in addition to the non-dinosaurian characters noted in the previous section.

References- Sharov, 1970. Svoyeobrazaya reptiliya iz nizhnego triasa Fergany. Paleontologicheskii Zhurnal. 1, 127-130.

Sharov, 1970. An unusual reptile from the Lower Triassic of Fergana. Paleontological Journal. 1, 127-130.

Haubold and Buffetaut, 1987. Une novelle interprétation de Longisquama insignis, reptile énigmatique du Trias supérieur d'Asie centrale. Comptes Rendus Académie des Sciences du Paris. 305, 65-70.

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

Benton, 1993. Reptilia. in Benton (ed). The Fossil Record 2. London. 681-715.

Jones, Ruben, Martin, Kurochkin, Feduccia, Maderson, Hillenius, Geist and Alifanov, 2000. Nonavian feathers in a Late Triassic archosaur. Science. 288(5474), 2202-2205.

Peters, 2000. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia. 106(3), 293-336.

Reisz and Sues, 2000. The "feathers" of Longisquama. Nature. 408(6811), 428.

Unwin, Alifanov and Benton, 2000. Enigmatic small reptiles from the Middle-Late Triassic of Kirgizstan. In Benton, Shishkin, Unwin and Kurochkin (eds). The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press, Cambridge. 177-186.

Jones, Ruben, Maderson and Martin, 2001. Longisquama fossil and feather morphology. Science. 291(5510), 1901-1902.

Prum, 2001. Longisquama fossil and feather morphology. Science. 291(5510), 1899-1900.

Reisz and Sues, 2001. Longisquama does not have feathers. Journal of Vertebrate Paleontology. 21(3), 92A.

Unwin and Benton, 2001. Longisquama fossil and feather morphology. Science. 291(5510), 1900-1901.

Feduccia, 2002. Birds are dinosaurs: Simple answer to a complex problem. The Auk. 119(4), 1187-1201.

Senter, 2003. Taxonomic sampling artifacts and the phylogenetic position of Aves. Unpublished PhD thesis. Northern Illinois University. 147 pp.

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

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

Peters, 2006. The other half of Longisquama. Prehistoric Times. 75, 10-11.

Renesto and Binelli, 2006. Vallesaurus cenensis Wild 1991, a drepanosaurid (Reptilia, Diapsida) from the Late Triassic of northern Italy. Rivista Italiana di Paleontologia e Stratigrafia. 112, 77-94

Martin, 2008. Origins of avian flight- a new perspective. Oryctos. 7, 45-54.

Voigt, Buchwitz, Fischer, Krause and Georgi, 2009. Feather-like development of Triassic diapsid skin appendages. Naturwissenschaften. 96, 81-86.

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