The Rise of the First Lords of the Sky – Dearc sgiathanach

In the previous article we discussed how pterosaurs were the first vertebrates to conquer powered flight. Yet their evolutionary achievements extend far beyond this milestone. Pterosaurs also produced the largest flying animals known in the history of life, with body masses approaching roughly 250 kg. Even today, no other flying animal has come close to reaching half of this size.

Because pterosaurs produced the largest flying animals ever known, researchers have long been fascinated by the evolution of gigantism in this group. Traditionally, it was thought that the trend toward larger body size began near the end of the Jurassic and into the Early Cretaceous, when pterodactyloids started to evolve. However, in 2022 researchers described a new pterosaur species from the Middle Jurassic, and this discovery demonstrated that the previous understanding was incomplete.

Dearc sgiathanach

Fossil, reconstruction, and body size of Dearc sgiathanach (圖片來源：Jagielska, N et al.(2022)，採用 CC BY 4.0 授權)

Dearc sgiathanach is a pterosaur species discovered in Middle Jurassic strata of Scotland in the United Kingdom. Its holotype specimen consists of a largely complete fossil skeleton.

Time period when Dearc sgiathanach lived（白稜製作）

Histological analysis of the bone tissue revealed only two lines of arrested growth, indicating that the individual was approximately two to three years old when it died. The bones contain extensive woven bone tissue penetrated by numerous blood vessels, which suggests that the individual was undergoing rapid growth at the time of death and had not yet reached adulthood. This interpretation is further supported by the presence of several skeletal elements that had not yet fully fused.

Phylogenetic analysis of the fossil indicates that Dearc sgiathanach belongs to the clade Angustinaripterini within the family Rhamphorhynchidae, a group of long-tailed non-pterodactyloid pterosaurs.

In pterosaurs, body mass generally correlates with wingspan, meaning wingspan is often the most reliable indicator of overall body size. The fossil specimen of Dearc is relatively large, but because several wing phalanges are missing, its wingspan cannot be measured directly. To estimate wingspan, researchers used datasets derived from numerous specimens of the rhamphorhynchid Rhamphorhynchus and the related pterosaur Dorygnathus, applying their anatomical proportions to the new species.

When skull measurements from Rhamphorhynchus were applied, the estimated wingspan of Dearc reached approximately 2.2 meters, and when humeral measurements were used the estimate increased to as much as 3.8 meters. By comparison, the largest known Rhamphorhynchus individuals had wingspans of only about 1.8 meters.

When the humeral proportions of Dorygnathus were applied, the wingspan estimate for Dearc was approximately 1.9 meters. The largest known Dorygnathus individuals had wingspans around 1.69 meters.

These analyses collectively indicate that Dearc sgiathanach was the largest Jurassic pterosaur known at the time the study was published. This conclusion is consistent with the fact that it possesses the largest skull and humerus yet recorded among Jurassic pterosaurs.

Because Rhamphorhynchus is represented by a much larger sample size and is phylogenetically closer to Dearc than Dorygnathus, researchers considered those estimates more reliable. Furthermore, the specimen died before reaching full maturity, meaning the animal would likely have grown even larger. For these reasons, researchers proposed that the wingspan of Dearc probably exceeded 2.5 meters and may have surpassed 3 meters.

Historically, explanations for the evolution of large body size in pterosaurs have focused on two main ideas. One hypothesis suggests that the appearance and diversification of birds occupied ecological niches previously held by pterosaurs, forcing pterosaurs to increase their body size to exploit new niches. Another hypothesis proposes that the more advanced anatomical structures of pterodactyloids allowed them to evolve larger bodies and more efficient flight.

However, the discovery of Dearc sgiathanach demonstrates that even non-pterodactyloid pterosaurs had already achieved large body sizes during the Middle Jurassic. Moreover, this occurred roughly 37 million years before the major radiation of birds, indicating that large body size in pterosaurs evolved much earlier than previously assumed.

Detailed anatomical study of the skeleton shows that although most of its features are characteristic of non-pterodactyloid pterosaurs, it also possesses certain traits typically associated with pterodactyloids. Earlier research proposed that the transition from non-pterodactyloids to pterodactyloids occurred rapidly, involving simultaneous transformation of much of the body plan—first the skull, followed by the torso, limbs, and tail. While the pterodactyloid-like features seen in Dearc do not contradict this interpretation, they indicate that some of these characteristics may have evolved convergently in multiple pterosaur lineages.

The discovery of Dearc also prompted researchers to re-examine fragmentary pterosaur fossils from other Middle Jurassic deposits in Britain. This review revealed at least seventeen isolated fossil specimens that would have belonged to animals with wingspans exceeding 1.7 meters, and several specimens may represent individuals with wingspans greater than 3 meters. These findings suggest that Jurassic pterosaurs were considerably larger than previously believed.

Finally, this reassessment of Middle Jurassic pterosaur material revealed clear evidence of taphonomic bias in the fossil record. The underlying cause of this bias remains uncertain and will require additional fossil discoveries and further research. Nevertheless, it is now evident that although Middle Jurassic pterosaur fossils are relatively rare, pterosaurs had already diversified extensively during this period and occupied a wide range of ecological niches.

Author: Bai Leng

Reference:

Jagielska, N., Sullivan, M. O., Funston, G. F., Butler, I. B., Challands, T. J., Clark, N. D. L., Fraser, N. C., Penny, A., Ross, D. A., Wilkinson, M., Brusatte, S. L. (2022). A skeleton from the Middle Jurassic of Scotland illuminates an earlier origin of large pterosaurs. Current Biology.