Sunday, 9 February 2020

Mimodactylus libanensis: A new species of Istiodactyliform Pterosaur from the Late Cretaceous Hjoûla Lagerstätte of Lebanon.

The Mesozoic Vertebrate palaeontology of Africa and Arabia is still largely veiled in mystery. Except for South Africa, where systematic studies, mainly in Triassic and Lower Jurassic continental deposits have provided a comparatively diverse Vertebrate fauna, information about the biota that lived during most of the Mesozoic Era in this region is extremely limited. This is particularly true for Pterosaurs, an extinct group of flying reptiles that includes the major powered flying Vertebrates for almost 160 million years. The main African records of this group are restricted to isolated elements from the Jurassic deposits of Tendaguru, Tanzania, the Late Cretaceous Kem Kem Beds and Ouled Abdoun deposits of Morocco. The most complete Pterosaur specimens from the Afro-Arabian region have been recovered from the Cenomanian (100.5-93.9 million year old) marine deposits of Lebanon. The first specimen was a partial left forelimb of a relatively small unnamed Ornithocheiroid Pterosaur from Hâqel Lagerstätte and the second a crushed skeleton formed mainly by two wings and the shoulder girdle of the Azhdarchoid Microtuban altivolans, from the coeval (equivalent in time) Hjoûla Lagerstätte.

In a paper published in the journal Scientific Reports on 29 November 2019, Alexander Kellner of the Laboratory of Systematics and Taphonomy of Fossil Vertebrates at the Universidade Federal do Rio de Janeiro, Michael Caldwell of the Department of Earth and Atmospheric Sciences at the University of Alberta, Borja Holgado, also of the Laboratory of Systematics and Taphonomy of Fossil Vertebrates at the Universidade Federal do Rio de Janeiro, and of the Institut Català de Paleontologia ‘Miquel Crusafont’ at the Universitat Autònoma de Barcelona, Fabio Dalla Vecchia, also of the Institut Català de Paleontologia ‘Miquel Crusafont’ at the Universitat Autònoma de Barcelona, Roy Nohra of Expo Hâqel, Juliana Sayão of the Laboratório de Biodiversidade do Nordeste at the Universidade Federal de Pernambuco, and Philip Currie, also of the Department of Earth and Atmospheric Sciences at the University of Alberta, describe a new species of Istiodactyliform Pterosaur from the Late Cretaceous Hjoûla Lagerstätte of Lebanon, which they consider to be the first complete and articulated skeleton including the skull and lower jaw from the Afro-Arabian region.

The new species is named Mimodactylus libanensis, where 'Mimodactylus' derives from the acronym MIM for the Mineral Museum of Beirut, which houses the specimen from which the species is described, plus '-dactylus' the Greek for digit (finger), a common suffix for Pterosaurs, which formed their wings from modified fingerbones, and 'libanensis' means 'from Lebanon'. The specimen from which the species is described is an almost complete skeleton, including the skull and lower jaw, recovered from the Hjoûla Lagerstätte of the Sannine Limestone, near the town of Hjoûla, 35 km north of Beirut and 10 km inland from Jbail, Lebanon.

Geographical location where the new Pterosaur, Mimodactylus libanensis, was collected. (a) Geological map of Lebanon. (b) Detail showing the location of the most important fossil Lagerstätten of Lebanon. (c) Position of Lebanon in the broad carbonate platform that surrounded the northern part of the Afro-Arabian region during the late Cenomanian. Abbreviations. a, Apulian Carbonate Platform (southern Italy); acp, Adriatic Carbonate Platform (Italy, Slovenia, Croatia); bd, Bei Daglari (Turkey); bih, Bihor Massif (Romania); bm, Bohemian Massif (Central Europe); et, Eastern Taurus (Turkey); g, Gavrovo (Greece); gm, Golija Massif (Serbia); inm, Insubrian Massif (Alps); sy, Seydisehir (Turkey); uks, Ukrainian Shield (Ukraine). The yellow star indicated on each map the location of the Hjoûla Lagerstätte (a), (b), whilst the red asterisk Lebanon(c). Kellner et al. (2019).

The specimen is well preserved with most parts of the skeleton articulated or only slightly displaced from their anatomical position. The skull and lower jaw are exposed in ventral view, with the occipital region and the craniomandibular articulation flattened. It is a comparatively small individual, with a wingspan of about 1.32 meters, and long wings, resulting in a high aspect ratio. Based on the unfused scapula and coracoid, pelvic elements and sacral vertebrae but fused dentaries at the symphysis, and dorsal vertebrae not fused into a notarium, it is likely that it was a very young animal at the time of death. 

Mimodactylus libanensis. (a) Photo and drawing of the complete specimen. (b) Close up of scapula and coracoid. (c) Detail of the wrist, showing the relation of the pteroid and the carpus. (d) Detail of the humerus. Scale-bars, (a): 50 mm; (b)-(d): 10 mm. Abbreviations. car: carpus; cdv: caudal vertebrae; cor: coracoid; cra: cranium; cs: cristospine; cv: cervical vertebrae; d: dentary; dcar: distal carpals; dpc: deltopectoral crest; dri: dorsal ribs; dv: dorsal vertebrae; fe: femur; hu: humerus; man: mandible; mcI-III: first to third metacarpals; mcIV: wing metacarpal; mtar: metatarsals; pcar: proximal carpals; ph1d4: first wing phalanx; ph2d4: second wing phalanx; ph3d4: third wing phalanx; ph4d4: fourth wing phalanx; ppu: prepubis; ptd: pteroid; ra: radius; sca: scapula; sri: sacral ribs; stp: sternal plate; sym: mandibular symphysis; tar: tarsus; te: teeth; ti: tibia; ul: ulna; the abbreviations ‘l’ and ‘r’ represents respectively left and right. Kellner et al. (2019).

The skull has a broad rostrum in dorsopalatal view, but not as rounded as in Istiodactylus, and also differing from other Istiodactyliforms. The rostral tip is pointed, unlike the rounded terminus of Istiodactylus. There are 11 and 10 cone-shaped teeth on each side of the upper and lower jaws, respectively, similar to Haopterus and Linlongopterus. The crowns of these teeth are labiolingually compressed (i.e. longer front-to-back than side-to-side), with a cingulum (a convex protuberance at the cervical third of the anatomic crown) as in Haopterus and other Istiodactyliforms. This cingulum was previously reported in Istiodactylidae and related taxa, but the teeth of this latter lineage is characterised by wide crowns, which have also a marked labiolingually compression.

Mimodactylus libanensis. (a) Skull and lower jaw. (b) Detail of the dentition. Scale bars, (a) 10 mm, (b) 1 mm. Abbreviations. hy: ceratobranchial I of the hyoid apparatus; man: mandible; odp: odontoid process; pl: palatine; pm: premaxilla; pplf: postpalatine fenestra; prid: palatal ridge; sym: mandibular symphysis; te: teeth; vo: vomer. Arrows point to the cingulum at the base of the teeth. Kellner et al. (2019).

The palate is concave and shows a small palatal ridge. Choanae (posterior nasal apertures) are large and divided by the vomers (facial bones). The postpalatinal fenestra (opening in the skull behind the palate) has an elongated egg-like shape as in the basal istiodactyliform Hongshanopterus. Ceratobranchials I  of the hyoid apparatus (bones which suspend the tongue and larynx) are fork-like, thin, and elongate elements. An odontoid process (ridge) is present at the tip of the lower jaw as in Istiodactylus latidens, but this process could also be recognised in Haopterus and Lonchodraco giganteus.

The dorsal vertebrae are exposed in ventral view and are not fused into a notarium. A total of 7 caudal vertebrae were identified, all of which lack a duplex centrum and decrease rapidly in size posteriorly, suggesting that this species had a short tail. The cristospine of the sternum is comparatively short and deep, similar to those of Nurhachius and Istiodactylus; the anterior portion of the sternum is more rounded in lateral view than that of Istiodactylids, being, in this respect, more similar to that of the Anhangueridae. The scapula is stout and shares with Istiodactylids and Anhanguerids a constricted shaft. However, it differs from both by being longer than the coracoid. The coracoid sternal articulation is slightly concave as in Haopterus and has a developed posterior expansion that is not present in Istiodactylids.

Detail of the dorsal vertebrae and dorsal ribs close to the right scapula and coracoid. Scale-bar is 10 mm. Abbreviations. cor.: coracoid; cv: cervical vertebrae; dri: dorsal ribs; dv: dorsal vertebrae; l: left; r, right; sca.: scapula. Kellner et al. (2019).

The humerus has a rectangular deltopectoral crest with an unusual straight distal margin and extends approximately 40% down the humerus shaft, more than in any other ornithocheiroid except for Pteranodon and related taxa. Mimodactylus has some wing elements longer relative to the humerus compared to istiodactylids, in particular the first and second phalanges. The distal portion of the last phalanx of the wing finger is curved as in most pterosaurs. The feet are relatively small, similar to Istiodactylids. The pteroid is quite large (longer than the humerus). This bone is clearly articulated with the proximal syncarpal and directed towards the body. There has been a long discussion about the position of this unique Pterosaur bone with the carpal elements, which is clearly settled in the present specimen whose forelimb bones are perfectly articulated.

Despite the incompleteness of the two other pterosaur specimens described from the upper Cenomanian of Lebanon, both of which lack cranial elements, those specimens can clearly be distinguished from Mimodactylus. The only complete bones of the Ornithocheiroid MSNM V 38818 are the wing metacarpal and the radius and ulna, whose proportions differ from those of Mimodactylus showing that the latter has a proportionally longer forearm. Furthermore, the diameter of the radius of MSNM V 3881 is less than half that of the ulna, contrary to the condition of Mimodactylus. The second specimen, the holotype of Microtuban altivolans, has a much shorter wing, a humerus with a different deltopectoral crest and a scapula that lacks the constricted shaft observed in Mimodactylus.

Another interesting feature of Mimodactylus is the dentition, which differs from that of most Ornithocheroids. As in Haopterus and Linlongopterus, the new species has cone-shaped dental crowns and they are confined to the anterior half part of the jaws. Such a configuration is present in other Istiodactyliforms and cannot be established in Lonchodraco giganteus due to preservation, which also present cone-shaped teeth. As in Haopterus but unlike Linlongopterus and Lonchodraco, teeth are characterised by a cingulum at the base of the crown, which is also present in the Istiodactylidae and closely related species. 

Teeth comparinson of Mimodactylus libanensis (a) with Haopterus gracilis (b). Scale-bar is 1 mm. Arrows point to the cingulum at the base of the teeth. Kellner et al. (2019).

Mimodactylus, however, lacks the lancet-shaped teeth with marked labiolingually compressed crowns that are diagnostic of the Istiodactylids. The new species also lacks the sharp carinae reported in Istiodactylus. The first upper tooth of Mimodactylus is comparatively small and has a sub-circular transverse section. It is followed by the largest teeth in the upper jaw, which have slight labiolingually compressed crowns with a cingulum, convex labial surfaces (the inner surfaces, closest to the tongue) and thin, lingually inclined, needle-like tips. This general morphology is present in the remaining teeth, also from the lower jaw. This kind of dentition is more similar to that of the basal Archaeopterodactyloids Pterodactylus and Germanodactylus rhamphastinus than to Istiodactylids and Ornithocheiroids. The sole other derived Pterodactyloid with a comparable dentition is Haopterus gracilis, first regarded as an Archaeopterodactyloid, later as an Ornithocheroid close to Istiodactylidae and even the sister taxa of Ornithocheiroidea. Kellner et al. recovered Haopterus at the base of Laceodontia as in more recent phylogenetic analyses, forming a clade with Mimodactylus libanensis.

An overview of Mimodactylus libanensis. (a) Phylogenetic relationships of Mimodactylus libanensis within Ornithocheiroidea. Colours show their continental origin: Afro-arabia (brown), Asia (orange), Europe (red), North America (blue), and South America (green). Outgroup relationships are not shown. Intermittent bars show uncertain temporal range. (b) Reconstructed silouette of Mimodactylus libanensis showing the long wings regards the body. Scale bar is 50 mm. Kellner et al. (2019).

Despite the inherent difficulties of establishing the diet of extinct vertebrates with no suitable modern analogues such as Pterosaurs, the following feeding habits have been proposed for derived Pterodactyloids, mainly based on their dentition (or absence of teeth) and the shape of their rostra: piscivory for Anhangueridae and their kin, Ikrandraco, Pteranodontidae, Nyctosauridae, Chaoyangopteridae, and Thalassodrominae; frugivory for Tapejarinae; durophagy for Dsungaripterus and related species; insectivory for Nemicolopterus; scavenging for Istiodactylus; cutting or ploughing through unconsolidated sediments for Argentinadraco; and terrestrial stalking for the long-necked Azhdarchids. The dentition of Mimodactylus differs from all of them, suggesting that this lineage of derived pterodactyloid had a different feeding habit.

Studies on the shapes of teeth of extant insectivorous Tetrapods emphasise that insectivorous species profit from having slimmer teeth that could be more easily used to breakdown arthropods due to the slight radius of curvature. Within the Pterosaur spectrum, the Anurognathids that have well-spaced and isodont teeth have been regarded as insectivores. Although wider, the tooth structure of Mimodactylus may suggest a similar feeding habit, allowing them to break up arthropod exoskeletons.

Aerial insectivory is closely linked to the ability to maneuver during flight. Extant vertebrate aerial insectivores exhibit short wings with low aspect ratios that allow them to be highly maneuverable in the air, contrary to Mimodactylus libanensis, which had long wings with high aspect ratio. In Mimodactylus, as open-sea flyers, the ability to maneuver during flight appears to be limited and it was likely high stable during flight as observed in Albatrosses and other birds. This might also have been the case for some large Pterosaurs such as Anhanguerians, Istiodactylids and Pteranodontians, which are considered to have conducted a dynamic soaring. Therefore, alternately to the insectivore hypothesis, Mimodactylus and their relatives might also have been capable of foraging for Decapod Crustaceans on surface waters, just like some Albatross species feed upon Caridean or Penaeid Shrimps. In addition, a broad rostrum and spaced but relatively robust and pointed teeth could be good tools to seize Shrimps in the water.

Life reconstruction of Mimodactylus libanensis. Julius Csotonyi in Kellner et al. (2019).

Observing the fossil content of the Hjoûla Lagerstätte as well as of other Cretaceous Lebanese Lagerstätten, Kellner et al. note that no Insects have been recovered so far. Even terrestrial plants are extremely rare at Hjoûla, suggesting that this Lebanese Lagerstätte was far away from emergent areas such as islands, with the continent several hundred kilometres away. On the other hand, Decapod Crustaceans are the most common invertebrates found in Hjoûla. No taphonomic bias has been detected to explain the absence of Insects, pointing to Fish and zooplankton as potentially the main local source of food for Pterosaurs. Mimodactylus libanensis also has a broad rostrum, which is consistent with a faunivorous feeding habit (primarily feeding on Crustaceans) as present in extant Ducks, Boat-billed Herons, and Shoebills. Thus, although insectivory cannot be ruled out, all available evidence suggests that Mimodactylus was feeding on Crustaceans.

This new pterosaur lived in archipelagos and scattered islands, which were present during the Late Cretaceous in the gigantic carbonate platform bordering the northern part of the Afro-Arabian region with the Neotethys. The discovery of Mimodactylus libanensis expands the spectrum of possible feeding strategies in derived Pterodactyloids, a group of fascinating volant Reptiles for which we still know very little.

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