Saturday, 11 September 2021

A Rhamphorhynchine Pterosaur from the Late Jurassic of Chile.

Pterosaurs first appeared in the Late Triassic, and persisted until the End of the Cretaceous. They are thought to have had a worldwide distribution for much of this time, however, their fossil record is somewhat patchy, which is thought to be largely due to the delicate nature of their skeletons, with the majority of known specimens coming from a relatively low number of fossil lagerstatten (sites producing fossils with exceptional preservation), the majority of which are in the northern hemisphere, particularly during the Jurassic, when almost all known specimens are from Germany, England, France, and China, the majority of them from the lithographic limestones of Solnhofen in Germany. All of these specimens would have lived on ancient Laurasia, while specimens from Gondwana, the Mesozoic supercontinent that made up the continents of the southern hemisphere, are much rarer. Some Pterosaurs are known from the Toarcian–Bathonian and Tithonian of Argentinian Patagonia, the lower Jurassic of Antarctica, and the Tithonian of Tanzania. In Chile, the Pterosaur fossil record compprises a few fragmentary specimens from the Lower Cretaceous, with two claims of Jurassic Pterosaurs from the Oxfordian Cerro Campamento Formation. However, one of these, initially interpretted as the incomplete skull if an indeterminate non-Pterodactyloid Pterosaur, was later re-interpretted as the dentary of a \Pachycormiform Fish. The other of these, however, appears to be the partial skeleton of a Rhamphorhynchine Pterosaur, a group widespread in the Jurassic of Europe, Asia, and North America.

In a paper published in the journal Acta Palaeontologica Polonica on 6 September 2021, Jhonatan Alarcón-Muñoz of Red Paleontológica and the Laboratorio de Zoología de Vertebrados at the Universidad de Chile, Rodrigo Otero, also of Red Paleontológica at the Universidad de Chile, and of Consultora Paleosuchus Ltda, and the Museo de Historia Natural y Cultural del Desierto de Atacama, Sergio Soto-Acuña, again of Red Paleontológica and of the Laboratorio de Ontogenia y Filogenia at the Universidad de Chile, and of KayTreng Consultores, Alexander Vargas, once again of Red Paleontológica at the Universidad de Chile, and Jennyfer Rojas and Osvaldo Rojas, also of the Museo de Historia Natural y Cultural del Desierto de Atacama, formerly describe this second Chilean Jurassic Pterosaur.

The specimen was collected at Cerritos Bayos, about 30 km to the west of the city of Calama in the Antofagasta Region of northern Chile. It was obtained from an outcrop of the Cerro Campamento Formation in the Atacama Desert, and was embedded in a concretion of yellowish-to-grey calcareous sandstone alongside numerous Ammonites, which have been refered to Subvinialesphinctes profetae and Euaspidoceras sp.. This concretion was set a few metres below the roof of the section, which is a distinctive blackish shale. The fossil-bearing horizon has been dated to the Callovian (Middle Jurassic, 166.1-163.5 million years ago), based upon the presence of the Ammonite 'Cosmoceras', although Alarcón-Muñoz et al. note that the presence of Subvinialesphinctes profetae and Euaspidoceras sp. makes it more likely that the section is Oxfordiam in age (Late Jurassic, 163.5-157.3 million years ago).

(A) Geographic location of the study area indicating the location of Cerro Campamento, the site where the discovery was made. (B) General scheme of the stratigraphic section which includes the Cerro Campamento Formation, indicating horizons with vertebrate remains and the Pterosaur materials studied. Alarcón-Muñoz et al. (2021).

The specimen (MUHNCAL.20165) was found by Osvaldo Rojas during fieldwork in 2009, and comprises a complete left humerus, two fragments of a diaphysis of a wing phalanx and a possible dorsal vertebra. These were all located within a single block, and revealed when the block was cracked open. All of the bones are consistent with an individual of the same size, and there are no duplicate bones, strongly suggesting that the remains represent a single medium-sized Pterosaur, which is assigned to the Subfamily Rhamphorhynchinae, but not to genus or species level, due to the fragmentary nature of the material.

Rhamphorhynchine Pterosaur (MUHNCAL.20165) from Cerritos Bayos, west Calama, Chile; Cerro Campamento Formation, middle Oxfordian. Vertebrae of indeterminate position (A₁) and schematic representation (A₂). Alarcón-Muñoz et al. (2021).

The single vertebra is incomplete and badly eroded. It is exposed in articular view, although due to the its eroded nature ir is unclear whether the front or back surface is exposed. It has a tall neural spine, with a preserved height of 6.3 mm, which is typical for Rhamphorhynchine Pterosaurs, particularly their cervical vertebrae, although some members of the group also have tall neural spines on their anterior dorsal vertebrae. The neural spine is compressed laterally, and appears to be slightly taller than the centrum, although it may have been taller still in life. One of the transverse processes is also preserved, extending about 4.5 mm from the neural arch, although again, this process is not complete, and may have been larger. The neural canal makes up about three fiths of the width of the centrum, and appears to be wider than high, though again poor preservation makes it hard to assess its true size and shape. The preserved height of the centrum is about 8.2 mm, and it appears to have had a semicircular articular contour, with a general morphology resembling a dorsal vertebra, although the poor preservation of the specimen means this cannot be asserted with any confidence.

An almost complete left humerus is also preserved. This is 71 mm long and 7.7 mm wide at its midpoint, giving it a length/width ration of 9.2. The head of the humerus is inclined backwards, which is a good indicator of the bone having come from a Pterosaur. An almost complete, dorsoventrally narrow, deltopectoral crest is present at the proximal end of the humerus and is extended anteriorly, projecting from the shaft of the humerus roughly perpendicularly. Both margins of the deltopectoral crest are approximately on the same level as the medial crest, something seen in some basal Pterodactyloids as well as some non-Pterodactyloid Pterosaurs. The deltopectoral crest is constricted at its base, and expands anteriorly, giving it a hatchet-like shape. This anterior end of this crest is slightly fractured, and joined to a carbonate layer in the rock, making it hard to determine its shape accurately.

Rhamphorhynchine Pterosaur  (MUHNCAL.20165) from Cerritos Bayos, west Calama, Chile; Cerro Campamento Formation, middle Oxfordian. Left humerus in dorsal (A₁, A₅), posterodorsal (A₂, A₆), posterior (A₃, A₇), and ventral (A₄, A₈) views, photographs (A₁–A₄) and explanatory drawings (A₃–A₈). Alarcón-Muñoz et al. (2021).

The hatchet or tongue-shaped form of the deltopectoral crest and its proximal location seen in specimen MUHNCAL.20165 is often considered diagnostic of hamphorhynchid Pterosaurs, and is seen in Rhamphorhynchus, Dorygnathus, Nesodactylus, Qinglongopterus, and Bellubrunnus. The shape of the crest in MUHNCAL.20165 is particularly similar to those of Rhamphorhynchus and Nesodactylus. The articular surface of the head of the humerus is badly eroded, but appears to show two bulges separated by a short groove, and to have a proximodorsally located triangular head, features also seen in Rhamphorhynchus and Nesodactylus

Comparison between the humeri of specimen MUHNCAL.20165 and other Rhamphorhynchids in dorsal (A)–(C), (E)–(G), ventral (D), (K)–(R), posterior (H), and anterior (I), (J) views. (A) MUHNCAL.20165, left humerus. (B Dorygnathus banthensis, right humerus. (C) Dorygnathus banthensis, left humerus. (D) Rhamphorhynchid Pterosaur (NHMUK PV R 40126b) right humerus. (E) Rhamphorhynchid Pterosaur (NHMUK PV R 40126c), right humerus. (F) Rhamphorhynchid Pterosaur (NHMUK PV R 28160a), left humerus. (G)  Rhamphorhynchid Pterosaur (OUM J.23043), left humerus. (H) Rhamphorhynchid Pterosaur (MUHNCAL.20165), left humerus. (I) Rhamphorhynchus muensteri, left humerus from. (J) Dorygnathus banthensis (SMNS 50164), right humerus. (K) Rhamphorhynchid Pterosaur (MUHNCAL.20165), left humerus. (L) Rhamphorhynchus muensteri, left humerus. (M) Qinglongopterus guoi (D3080/1), left humerus. (N) Bellubrunnus rothgaengeri (BSP–1993–XVIII–2), left humerus. (O) Sericipterus wucaiwanensis (IVPP V14725), right humerus. (P) Rhamphorhynchus etchesi (MJML K-1597), right humerus. (Q) Rhamphorhynchid Pterosaur (PRC 64), right humerus. (R) Nesodactylus hesperius (AMNH 2000), left humerus. Scale bars: (A)–(C), (E)–(M), (O)–(R), 10 mm; (D), (N), 5 mm. Alarcón-Muñoz et al. (2021).

The way in which the humerous is preserved, with its posterior surface exposed and its antirior surface covered, it is impossible to tell if there are pneumatopores piercing the anterior surface of its proximal end, a feature absent in basal Pterosaurs, including Rhamphorhynchus muensteri, and present in Pterodactyloids. 

The medial (or ulnar) crest probably served as an attachment for the subcoracoscapularis muscle. In MUHNCAL.20165 this has a slightly rounded distal edge which forms an obtuse angle with the diaphysis, giving it an almost square shape. It is shorter than seen in any species currently assigned to the genera Rhamphorhynchus, Dorygnathus, or Nesodactylus. It also differs from Nesodactylus in that its posterior form does not form an accute angle and it is more extended proximo-distally.

The humerous is preserved in three dimensions, and does not appears to have been distorted during preservation. The diaphysis of MUHNCAL.20165 appears robust and straight when viewed from the posterior, but is nevertheless more slender than those of Rhamphorhynchus muensteri or Rhamphorhynchus etchesi. Viewed dorsally it appears curved to the anterior; this is common in non-Pterodactyloid Pterosaurs, although the extent of the curvature is highly variable. The shart is also notably curved, as is seen in the Rhamphorhynchids Rhamphorhynchus, Dorygnathus, and Nesodactylus, as well as some unnamed specimens assigned to the group. 

The distal epiphysis of MUHNCAL.20165 is expanded lateromedially, as is seen in Rhamphorhynchus muensteri, Dorygnathus banthensis, and an unnamed Rhamphorhynchid from the Oxford Clay of England. The ectepicondyle protrudes dorsally and is triangual in shape, it is more proximal than the entepicondyle, which does not protrude ventrally. The ventral part of the distal epiphysis is more extended than the dorsal part, contrary to the position in Rhamphorhynchus, in which the two halves are roughly level.

Also present are two fragments of part of the shaft of a wing phalanx. This is closely associated with the humerus, possibly suggesting they came from the same wing, although the fragmentary nature of the material makes this impossible to assert with any confidence, nor to assess which of the four wing phalanxes of the Pterosaur it might have come from. The combined length of the two fragments is 161.7 mm, longer than the first wing phalanx of Sericipterus wucaiwanensis, but shorter than most other know Rhamphorhynchid wing phalanxes; since the total length of the bone is unknown, little more can be inferred from this.

Unnamed Rhamphorhynchid (MUHNCAL.20165) from Cerritos Bayos, west Calama, Chile; Cerro Campamento Formation, middle Oxfordian. Fragments of a wing phalanx in dorsal (A₁) and posterior (A₂) views. Alarcón-Muñoz et al. (2021).

The phalanx has a longitudinal furrow flanked by two asymmetric crests, with the dorsal crest being more prominent than the ventral crest. Such furrows are considered diagnostic of Rhamphorhynchids, although it has also been observed in the Dimorphodontid Caelestiventus hanseni from the Upper Triassic from North America, although this species (and the family Dimorphodontidae in general) is to far removed chronologically to make it a likely close relative of MUHNCAL.20165.

The material assigned to MUHNCAL.20165 is somewhat limited, but clearly derives from a Pterosaur, and shows several diagnostic features associated with Rhamphorhynchids, (although there is currely some debate about the phylogeny of the Rhamphorhynchidae, with disputes about whether several species should be included in the group or not). The Rhamphorhynchidae is further divided into two subclades, the Rhamphorhynchinae and Scaphognathinae; of these MUHNCAL.20165 shows closer affinities to species placed within the Rhamphorhynchinae.

The Family Rhamphorhynchidae is well represented in the Upper Jurassic faunas of Laurasia, but less well known in the Jurassic of Godwana. Specimens described from the Middle and Upper Jurassic of India, the Upper Jurassic of Tanzania, and the Lower–Middle Jurassic of Argentinean Patagonia have subsequently been re-assigned to other groups of Pterosaurs or even non-Pterosaur Vertebrates. More recently, teeth assigned to Rhamphorhynchids have been described from the Bathonian Guelb el Ahmar nnd Upper Jurassic–Lower Cretaceous of Ksar Metlili in Morocco.

Thus, if MUHNCAL.20165 is a Rhamphorhynchid, then it is the first known skeletal material from the group recorded from Gondwana. It is also notable that Pterosaurs from the Oxfordian are rare in general, and particularly so in Gondwana, making a specimen expanding the range of the Rhamphorhynchidae during this stage particularly significant, although not totally unexpected, as Pterosaurs could fly and Gondwana and Laurasia were still connected at least some of the time during the Middle and Late Jurassic.

Most Rhamphorhynchids are thought to have inhabited marine and coastal ecosystems (some Chinese specimens have been found on continental deposits, indicating they probably lived far from shore), and MUHNCAL.20165 was derived from a marine sediment, consistant with it being assigned to this group. At least one specimen, Rhamphorhynchus muensteri from the Solnhofen Limestone of Germany, has been found with Fish preserved in its stomach, indicating a marine-derived diet. 

In the Oxfordian, Laurasia and Gondwana had roughly continuous coats to the east and west, and were linked by the narrow seaways of the Caribbean Corridor and Trans-Erythrean Corridor. The Caribbean Corridor in particular would have provided a continuous system of shallow marine and coastal ecosystems linking Laurasia and Gondwana, which was geographically close to Chile. This corridor has been used to explain a number of previous faunal links between Europe and South America at this time, and a Rhamphorhynchid, Nesodactylus hesperius, has previously been recorded from Cuba, which would have been within the Caribbean Corridor at the time.


Simplified map of the world during the Oxfordian. The dashed line represents the hypothetical route that would have allowed the dispersal of marine Vertebrates and Invertebrates between Tethys and South America during the Late Jurassic (Caribbean Corridor). The finds of Oxfordian Rhamphorhinchids: MUHNCAL.20165, Cerro Campamento Formation, Chile (circle); Cacibupteryx caribensis and Nesodactylus hesperius, Jagua Formation, Cuba (square); unnamed Rhamphorhinchid, Oxford Clay Formation, UK (star); Qinglongopterus guoi, Tiaojishan Formation, China (triangle). Alarcón-Muñoz et al. (2021).

The new specimen, MUHNCAL.20165, represents the oldest known Pterosaur from Chile, and the first specimen confidently assigned to the Rhamphorhynchinae from Gondwana, although the sparse nature of the material makes any more detailed analysis difficult without further material coming to light.

The Cerro Campamento Formation has produced a number of significant Vertebrate fossils in recent years, although most of these have been Marine Reptiles. These specimens have supported the existence of a biogeographical connection between South America and Europe in the Late Jurassic. MUHNCAL.20165 presents the first evidence of a non-marine Vertebrate following the same biogeographical distribution route. The Cerro Campamento Formation produces numerous fossil-rich concretions, which are likely to continue to add to this picture.

See also...

Online courses in Palaeontology.  

Follow Sciency Thoughts on Facebook.

Follow Sciency Thoughts on Twitter