Protoavis

Protoavis (meaning "first bird") is a problematic taxon of archosaurian whose fossils have been recovered from Late Triassic Norian stage deposits near Post, Texas. Much controversy remains over the animal, with many different interpretations of what Protoavis actually is existing. When it was first described, the fossils were ascribed to a primitive bird which, if the identification is valid, would push back avian origins some 60-75 million years. The original describer of Protoavis texensis, Sankar Chatterjee of Texas Tech University, interpreted the type specimen to have come from a single animal, specifically a 35 cm tall bird that lived in what is now Texas, USA, between 225 and 210 million years ago. Though it existed far earlier than Archaeopteryx, its skeletal structure is allegedly more bird-like. Protoavis has been reconstructed as a carnivorous bird that had teeth on the tip of its jaws and eyes located at the front of the skull, suggesting a nocturnal or crepuscular lifestyle. Reconstructions usually depict it with feathers, as Chatterjee originally interpreted structures on the arm to be quill knobs, the attachment point for flight feathers found in some modern birds and non-avian dinosaurs. However, re-evaluation of the fossil material by subsequent authors such as Lawrence Witmer have been inconclusive regarding whether or not these structures are actual quill knobs.

However, this description of Protoavis assumes that Protoavis has been correctly interpreted as a bird. Almost all paleontologists doubt that Protoavis is a bird, or that all remains assigned to it even come from a single species, because of the circumstances of its discovery and unconvincing avian synapomorphies in its fragmentary material. When they were found at the Tecovas and Bull Canyon Formations in the Texas panhandle in 1984, in a sedimentary strata of a Triassic river delta, the fossils were a jumbled cache of disarticulated bones that may reflect an incident of mass mortality following a flash flood.

Description
Protoavis is usually depicted as being a bipedal archosaur, similar to several poposaurs and rauisuchids that may have been partially or habitually bipedal and lived during roughly the same time as Protoavis.[2] In a description published by Sankar Chatterjee, structures were identified as quill knobs,[3] although there has been debate as to whether these are actually quill knobs or not.

Skull and braincase
The braincase of Protoavis is similar in some respects to Troodon, with an enlarged cerebellum that shifted the optic lobes ventrolaterally, and also has a large floccular lobe.[6] The inner ear is also pretty similar and bird-like in both taxa. The canalicular systems and the cochlear process differ in both taxa, and the vestibular region is relatively small and located in a ventral position to most of the anterior and posterior semicircular canals. The anterior semicircular canal is significantly longer then the others, and the cochlear process is a relatively long, vertically oriented tube.[6]:2244 However, Protoavis is also remarkedly non-bird like in that it possess only a single exit for the trigeminal.[6]:2244 However, these characters are not robust enough to identify Protoavis as a bird.[6]

The skull has an extremely narrow parietal with block like dorsal aspect, very broad, T-shaped frontals that form the "lateral wings" that Chatterjee applies to the lack of postorbitals. There are short curved ulnae with olecranon processes, and a possible scapula with bent shaft, and the cervicals have profiles and aspects to their exterior that are very similar to the Megalancosaurus cervical series. All the cervicals but the most posterior and axis/atlas have hypopophyses and those triangular neural spines; all characteristics that have been described in Megalancosaurus.[7][Note 1] This suggests that portions of Protoavis may be drepanosaurid in nature.[7]

Chatterjee presents the skull of Protoavis as complete, although only the caudal aspect of the cranium is represented in the available fossils.[4][8][9] Chatterjee argues that the temporal region displays a streptostylic quadrate with orbital process for attachment of the M. protractor pterygoidei et quadrati, with associated confluence of the orbits with the temporal fenestrae, thus facilitating prokinesis. He further asserts ornithurine synapomorphies for the braincase of Protoavis including the structure of the otic capsule, the widespread pneumatization of the braincase elements, a full complement of tympanic recesses, and the presence of an epiotic.

Of this material, only the quadrate and orbital roof, in addition to limited portions of the braincase are preserved with enough fidelity to permit any definitive interpretation.[10][9] The quadrates of TTU P 9200 and TTU P 9201 are not particularly alike; a fact not easily explained away if the material is conspecific, as Chatterjee insists.[9] There does not appear to be an orbital process present on either bone, and the modifications of the proximal condyle permitting wide range of motion against the squamosal, are not readily apparent. Furthermore, the quadratojugal and jugal appear far more robust in the Protoavis specimens themselves, than represented by Chatterjee.[11][1] The size and development of the quadratojugal seems to contradict Chatterjee's assertion that this bone contacted the quadrate via a highly mobile pin joint.[10][4][9] These data render the assertion of prokinesis in the skull of Protoavis questionable at best, and it seems most parsimonious to conclude that the specimen displays a conventional opisthostylic quadrate.

The braincase is where Protoavis comes close to being as avian as Chatterjee has maintained. The otic capsule in allegedly organized in avian fashion, with three distinct foramina arranged as such: fenestra ovalis, fenestra pseudorotunda, and the caudal tympanic recess, with a bony metotic strut positioned between the fenestra pseudorotunda and caudal tympanic recess.[5][11] The claim that the full complement of tympanic recesses seen in ornithurines, are similarly observed in Protoavis is questionable, as the preservation of the braincase is not adequate to permit concrete observations on the matter. Chatterjee omits in his 1987 account of the braincase, the presence of a substantial post-temporal fenestra,[12] which in all Aves (including Archaeopteryx), is reduced or absent altogether,[13][9] and the lack of a pneumatic sinus on the paroccipital.[13] Furthermore, the braincase possesses multiple characters symplesiomorphic of Coelurosauria, including an expanded cerebellar auricular fossa, and a vagal canal opening into the occiput.[14] These data suggest that in the braincase attributed to Protoavis is the earliest known record of a coelurosaurian theropod, as opposed to that of an ornithothoracine bird. What is preserved of the preorbital skull curiously lacks apomorphic characters to be expected in a specimen, which is allegedly more closely allied to Ornithurae than is Archaeopteryx lithographica. Most telling is the complete absence of accessory fenestrae in the antorbital fossa, leading to maxillary sinuses.

post-cranial anatomy
The post-cranial remains are as badly preserved, or worse, than the cranial elements, and their interpretation by Chatterjee[11] are in many cases unsubstantiated or speculative. Of the postcranial skeleton, Chatterjee has isolated the axial skeleton as displaying a suite of avian characters, including heterocoelus centra, hypapophyses and reduction of the neural spines. First and foremost, the preservation quality of the vertebrae is poor. While the centra are modified, they do not appear to be truly heterocoelus.[9] The presence of incipient hypapophyses in and of itself might be considered indicative of avian affinity, but their poor development and presence on vertebrae otherwise thoroughly non-avian, is most parsimoniously regarded as mild convergence until further material should be brought to light. The reduction of the neural spines is questionable.

Curiously, Gregory Paul has noted that the cervicals of Protoavis and drepanosaurs are astonishingly similar, such they are hardly distinguishable from one another.[9]:Fig. 10.7Ba Considering the modification of the drepanosaur neck for the purposes of snap-action predation, it becomes more likely that superficial similarities in the cervicals of both taxa are in fact only convergent with Aves.[9] Chatterjee does not identify the remaining vertebrae as particularly avian in their osteology.

Pectoral girdle
The pectoral girdle is discussed by Chatterjee as being highly derived in Protoavis, displaying synapomorphies of Ornithothoraces, including the presence of a hypocleidium-bearing furcula, and a hypertrophied, carinate sternum. Chatterjee's interpretation of the fossils identified as such in his reviews of the Protoavis material[11] are open to question due to the preservation quality of the elements and as of this time, it is not clear whether either character was in fact present in Protoavis.[10] The glenoid appears to be oriented dorsolaterally permitting a wide range of humeral movement. Chatterjee implies that this is a highly derived trait which allies Protoavis to Aves,[11] but why this should be so is not clearly discussed in the descriptions of the animal. In and of itself, the orientation of the glenoid is not a sufficient basis for placing Protoavis within Aves. The scapular blade is far broader than illustrated by Chatterjee in his 1997 account,[11] and not particularly avian in its gross form.[9] The coracoid, identified by Chatterjee as strut-like and retroverted, is, like the supposed furcula and sternum, too poorly preserved to permit accurate identification. Moreover, the original spatial relationship of the alleged coracoid to the scapula is entirely unknown.[4][9] Uncinate processes and sternal ribs are missing.

Pelvic girdle
Chatterjee asserts that the pelvic girdle is apomorphic comparative to archaic birds and displays a retroverted pubis, fusion of the ischium and ilium, an antitrochanter, and the presence of a renal fossa. The pubis does appear to display opisthopuby, although this has yet to be verified. The alleged fusion of the ischium and ilium into an ilioischiadic plate is currently not substantiated by the fossils at hand, despite Chatterjee's auspicious illustration to the contrary in The Rise of Birds.[15][8][9] At this time the pelvic girdle is not sufficiently well preserved to ascertain whether or not a renal fossa was present, although as no known avian from the Mesozoic displays a renal fossa, it is not clear why Protoavis should, even if it is ornithothoracine.[14] Similarly, it is unclear if the alleged antitrochanter has been correctly identified as such.

Arms and legs
The manus and carpus are among the few areas of the Protoavis material, which are well preserved, and they are astonishingly non-avian. The distal carpals, while long, are in no way similar to those observed in the urvogel or other archaic birds. There is not semilunate element, and the structure of the radiale and ulnare would have limited the flexibility of the wrist joint.[11][9] The manus is not tridactyl, and metacarpal V is present. In even the most basal avialian, Archaeopteryx, there is no vestige of the fifth metacarpal and its presence in Protoavis; seems incongruous with the claim that it is a bird, let alone one more derived than Archaeopteryx.

Chatterjee claims that the humerus of Protoavis is "remarkably avian",[11]:53 but as in all matters with the fossils referred to this taxon, accurate identification of the elaborate trochanters, ridges, etc., attributed to the humerus by Chatterjee is impossible at this time. Interestingly, the expanded distal condyles, which appear to be present in the humerus of Protoavis and enlarged deltopectoral crest, are congruent with the morphology of ceratosaur humeri, as is the apparent presence of a distal brachial depression.[16]

The femur of Protoavis is astonishingly like that of a ceratosaur. The proximal femur displays a trochanteric shelf caudal to the lesser and greater trochanters, a feature synapomorphic of Ceratosauria[17][16] Further similarities between the proximal humerus of Protoavis and that of ceratosaurs are found in the shared presence of an enlarged obturator ridge, whose morphology in Protoavis is again, uncannily like that observed in robust ceratosaurs, e.g., Syntarsus.[17] The resemblance between the femur of Protoavis and that of a ceratosaur becomes ever more pronounced at the distal end of the bone. Both share a crista tibiofibularis groove, a synapomorphy of ceratosaurs separating the medial and lateral condyles.[17][16][11]

The tibia of Protoavis allegedly possesses both a lateral and cranial cnemial crest, though the validity of this claim is subject to question due to the preservation quality of the material. The fibula is continuous to the astragalocalcaneal unit. A tibiotarsus is absent, unusual considering Chatterjee's claims for the ornithurine affinity of Protoavis, as is a tarsometatarsus.[18][9] The ascending process of the astragalus is reduced, a character entirely incongruous with a highly derived status for Protoavis. Curiously, such abbreviation of the ascending process is found in ceratosaurs, and in its general osteology, the Protoavis tarsus and pes, is quite similar to those of ceratosaurs. Chatterjee's restoration of the hallux as reversed is nothing more than speculation, as the original spatial relationships of the pedal elements are impossible to ascertain at this time.

Quill knobs
Reconstructions usually depict it with feathers, as Chatterjee originally interpreted structures on the arm to be quill knobs, the attachment point for flight feathers found in some modern birds and non-avian dinosaurs. However, re-evaluation of the fossil material by subsequent authors such as Lawrence Witmer have been inconclusive regarding whether or not these structures are actual quill knobs.[9]

In his 1997 account, Chatterjee infers the presence of feathers from alleged quill knobs on the badly smashed ulna and metacarpals III and IV, and infers the presence of remiges from such structures(though he does caution that this is uncertain).[11] As is the case with the alleged quill knobs on the ulna, the metacarpal structures appear to be attributable to post-mortem damage.[9] Moreover, the thumb, unlike the case in all birds, is not medially divergent. Considering how poorly preserved the ulna is, it is entirely premature to make any definitive conclusions as to the presence of quill knobs until such time as more adequate material becomes available. Upon further examination of the material no structures were isolated that could be deemed as homologous to remigial papillae.

History of discovery
Archosaurs discoveries are comparatively abundant in Texas, and have been recovered in some quantity since E. D. Cope worked the redbeds of the panhandle over a century ago.[20] The holotype specimen of Protoavis (TTU P 9200), the paratype (TTU P 9201), and all referred materials,[Note 2] were discovered in the Dockum Group, from the panhandle of Texas. The Dockum dates from the Carnian through the early Norian, in the terminal Triassic and is composed of four units of decreasing age: the Santa Rose Formation, the Tecovas Formation, the Trujillo Formation, and the Cooper Canyon Formation, and the Bull Canyon Formation.[21][11] Many skeletal elements and partial elements of Protoavis were collected from the Post (Miller) Quarry of the Bull Canyon Formation in the 1980s and other specimens referred to Protoavis were collected from the underlying Kirkpatrick Quarry of the Tecovas Formation.[1] The specimens altogether consists of a partial skull and postcranial remains belonging to possibly several large individuals.[5][11] The bones were completely freed of the surrounding matrix, and some were heavily reconstructed and the identification of some of the elements have been questioned by other paleornithologists and paleontologists.

The type material was collected from mudstone deposits in June of 1973 and initially identified as a juvenile Coelophysis bauri.[11] The level of the Dockum group from which the Protoavis material was recovered, was most likely deposited in a deltaic river system. The bone bed excavated by Sankar Chatterjee and his students of Texas Tech University, in which Protoavis was discovered, likely reflects an incident of mass mortality following a flash flood.[11][22] Chatterjee, who first described Protavis, has assigned the binomial Protoavis texensis ("first bird from Texas") to the small cache of bones, allegedly conspecific. He interpreted the type specimen to have come from a single animal, specifically a 35 cm tall bird that lived in what is now Texas, USA, between 225 and 210 million years ago.

Due to the nature of the bones being jumbled into sandstone nodules, and completely disarticulated, it has been suggested that Protoavis was reworked from later sediments.[23] However, a basic stratigraphic principle, the "principle of inclusions", is a special case of the principle of cross-cutting relationships. It states that rock has to exist before it can be included in other sedimentary rock. Reworking is the process of weathering fossils or rock containing fossils out of rocks already present, transporting them, and redepositing them in sediments which are later lithified as new sedimentary rocks. Since the Jurassic rocks occurred after the Triassic sediments of the Dockum Group, they could not have been reworked into the Dockum sediments as inclusions.