Dearc

Dearc sgiathanach is a genus and species of large-bodied rhamphorhynchid pterosaur from the Middle Jurassic (Bathonian stage) Lealt Shale Formation on the Isle of Skye, Scotland, known from a single well-preserved three-dimensional skeleton representing a juvenile or subadult individual.^[1] This specimen, with a skull length of 222 mm and humerus length of 112–118 mm, indicates a wingspan exceeding 2.5 m and potentially reaching over 3 m in adulthood, establishing Dearc as one of the largest known pterosaurs from the Jurassic period.^[1]^[2] The holotype (NMS G.2021.6) was discovered in 2017 by amateur fossil collector Amelia Penny at Rubha nam Brathairean, a locality yielding lagoonal bioclastic limestones approximately 170 million years old.^[3] Housed at the National Museum of Scotland, the fossil includes a partial skeleton with skull, vertebrae, ribs, sternum, wing elements, and a brain endocast, marking it as the best-preserved pterosaur specimen from the UK in over a century.^[3]^[2] Notable features include sharp, enamel-covered teeth suited for fish-snatching, large optic lobes suggesting keen eyesight, and highly recurved unguals indicating possible scansorial or arboreal behaviors.^[3]^[2] Phylogenetically, Dearc belongs to the clade Angustinaripterini within Rhamphorhynchidae, a group of basal, long-tailed pterosaurs, and exhibits derived traits convergent with later pterodactyloids, such as exapophyses on cervical vertebrae associated with larger body sizes.^[1]^[2] Its discovery challenges previous assumptions about pterosaur size evolution, demonstrating that large-bodied forms (>2.5 m wingspan) originated in the Middle Jurassic, well before the diversification of pterodactyloids in the Late Jurassic and Cretaceous.^[1] This finding highlights a previously unrecognized diversity of Middle Jurassic pterosaurs and underscores the Isle of Skye's importance as a key site for understanding early pterosaur evolution.^[4]^[5]

Discovery and naming

Geological context

The holotype specimen of Dearc sgiathanach was recovered from the Lonfearn Member of the Lealt Shale Formation, part of the Great Estuarine Group, exposed at Brothers' Point on the Isle of Skye, Scotland.^[1] This formation dates to the Bathonian stage of the Middle Jurassic, approximately 167 million years ago.^[1] The depositional environment of the Lealt Shale Formation represents a marginal marine to nearshore lagoonal setting characterized by fluctuating salinity and periodic subaerial exposure, with sedimentation dominated by low-energy, fine-grained bioclastic limestones and mudstones rich in bivalves such as Neomiodon, ostracods, and conchostracans.^[1] These conditions, including biologically active mudflats and anoxic bottom waters evidenced by diagenetic pyrite formation, promoted exceptional three-dimensional preservation of articulated skeletons with minimal compaction or fracturing.^[1] The specimen lies within a well-sorted limestone layer overlying subaerially exposed mudstones bearing dinosaur trackways.^[1] Associated fauna in the Lealt Shale Formation includes fish remains from bone beds, indicating marine influences, as well as crocodylomorph elements reported from equivalent strata on the Isle of Eigg.^[6] Theropod tracks, alongside ornithopod and possible sauropod impressions, occur in the same low-energy coastal ecosystem, suggesting a diverse vertebrate community adapted to dynamic shoreline habitats.^[6]

History of discovery and description

The fossil specimen of Dearc sgiathanach was discovered in 2017 by amateur collector and PhD student Amelia Penny during a field trip to the Isle of Skye organized by the University of Edinburgh.^[3]^[5] The find occurred at Brothers' Point on the island's Trotternish Peninsula, within the Lealt Shale Formation of the Bathonian stage of the Middle Jurassic.00135-X) Following its discovery, the specimen underwent preparation and initial study at National Museums Scotland in Edinburgh, where it was accessioned as the holotype NMS G.2021.6.1-4.00135-X) This partial skeleton, comprising elements including the skull, vertebrae, and limb bones, is preserved in three dimensions, making it one of the most complete Middle Jurassic pterosaur fossils known.00135-X)^[3] Formal description and naming occurred in 2022 by Natalia Jagielska and colleagues in a paper published in Current Biology, establishing Dearc sgiathanach as a new genus and species within the non-monofenestratan pterosaurs.00135-X) The generic name Dearc derives from Scottish Gaelic for "reptile" or "serpent," while the specific epithet sgiathanach means "winged," honoring the Scottish heritage of the discovery site.^[7]00135-X) Subsequent research includes a 2025 detailed osteological analysis by Jagielska and colleagues in BMC Ecology and Evolution, which incorporated high-resolution X-ray microtomography and manual segmentation for 3D modeling, with datasets made available via MorphoSource (project ID 000420007).^[2]^[8] This study retained the initial 2022 phylogenetic placement of Dearc within the clade Angustinaripterini while providing enhanced anatomical resolution.^[2]

Description

Size and ontogeny

Dearc sgiathanach is estimated to have had a wingspan of 2.5–3 meters, making it one of the largest known Jurassic pterosaurs (and the largest from the Middle Jurassic).^[9]^[10] This estimate is derived from scaling the humerus length—measured at 112 mm for the right humerus and 118 mm for the left—against related rhamphorhynchine pterosaurs such as Rhamphorhynchus and Dorygnathus.^[9] The skull measures 222 mm in length, while the total body length, including the elongate tail, is approximately 1.6 meters.^[9] These dimensions exceed those of previous Middle Jurassic records, such as Campylognathoides, which had a wingspan of around 1.7–2 meters.^[9] The specimen represents a juvenile or subadult individual, as evidenced by several indicators of ongoing growth. Unfused elements, including the jugal with the lacrimal and limited fusion of the sacral vertebrae, suggest osteological immaturity.^[9] Bone histology reveals primary fibrolamellar bone tissue with two lines of arrested growth (LAGs) and dense vascularization, but no external fundamental system, confirming active growth at the time of death.^[9] This ontogenetic stage implies that Dearc had the potential to reach even larger adult sizes, further highlighting the early evolution of giant pterosaurs in the Jurassic.^[9]

Cranial anatomy

The skull of Dearc sgiathanach measures 222 mm in length and exhibits a low, elongate profile with a height-to-length ratio of less than 0.2, resulting in a dorsoventrally compressed structure approximately 35 mm tall at its maximum preserved height.^[9] The antorbital fenestra is notably large, spanning 50 mm and comprising over 20% of the total skull length, with the preorbital region exceeding the postorbital region in length, a configuration typical of basal non-pterodactyloid pterosaurs but accentuated in this specimen.^[9] This fenestra occupies more than 80% of the orbit's dorsoventral height, contributing to the skull's lightweight construction while maintaining structural integrity for flight-related stresses.^[9] The palate displays distinctive features, including trident-shaped vomers formed by three cylindrical rods that converge anteriorly in a precapillary contact, representing an autapomorphy of the species.^[9] These vomers fork medially to divide the broad, heart-shaped choanae, which measure 45 mm in length and 17 mm in width, facilitating substantial airflow through the nasal passages.^[11] The pterygoids are robust, with their plates defining key palatal fenestrations such as the postpalatine (6 mm anteroposteriorly by 5 mm transversely), suborbital (16 mm by 9 mm), and subtemporal (25 mm by 9 mm) openings; an ectopterygoid extension forms the elevated ventral border of the postpalatine fenestra, suggesting reinforced palatal musculature to compensate for relatively weak jaw adductors.^[11] The jaws feature an elongated mandibular symphysis that exceeds 20% of the total mandible length, providing a reinforced anterior margin for prey capture.^[9] Dentition includes approximately 13 teeth, with prominent elongate fangs up to 19 mm long at the premaxillary tip for initial grasping, transitioning to smaller conical peg-like teeth (about 3.8 mm in basal diameter) spaced along the maxilla and mandible; enamel wear and cracking on these teeth indicate adaptation for handling hard-surfaced prey.^[11] The lower jaw is straight and slender, approximately 10 mm thick, with 6–7 alveolar pairs per ramus, emphasizing a grasping rather than crushing function.^[11] Sensory structures are highlighted by the large choanae, which serve as major internal nasal openings, and endocranial features revealed through μCT scanning, including anteroposteriorly expanded optic lobes that are larger and more dorsally exposed than in other basal pterosaurs, indicating enhanced visual processing capabilities.^[9] Although no sclerotic ring is preserved, the orbit's proportions and optic lobe development suggest the potential for a robust eye-supporting structure suited to aerial environments.^[9]

Postcranial skeleton

The postcranial skeleton of Dearc sgiathanach is well-preserved and articulated, providing insights into its transitional morphology between basal and more derived pterosaurs. The axial skeleton includes a relatively short neck comprising six postaxial cervical vertebrae, with a total length of approximately 135 mm; the third cervical vertebra measures 30.4 mm in length and 16.6 mm in width, yielding a length-to-width ratio of 1.8, while posterior cervicals exhibit ratios as low as 1.2. These vertebrae feature elongate proportions and small exapophyses, up to 5 mm long and 8 mm wide on the third cervical, suggesting early adaptations for neck flexibility. The dorsal series consists of nine vertebrae totaling about 235 mm in length, with the anterior-most measuring 19 mm long and 12 mm wide; these display exapophyses indicative of transitional traits between non-pterodactyloid and pterodactyloid forms. The tail is elongate, with around 10 preserved caudal vertebrae spanning approximately 190 mm, supported by interlocking zygapophyses and chevrons, though the distal portion is incomplete; anterior caudals are notably thin, with a maximum width of 3.4 mm.^[9] The pectoral girdle is robust, featuring a heart-shaped sternum that measures 34 mm wide and 23 mm long, with its anterior margin deflected outward—an autapomorphy unique to Dearc. The coracoids are sturdy, with the left measuring 40 mm long and 5.5 mm wide, and the right reaching 51.6 mm in length while flaring distally from 5 mm to 11 mm; they articulate with partially preserved, straight scapulae that lack complete measurements but show a robust proximal end. Wing elements are elongated and strongly built, consistent with the animal's large size. The humerus is prominent, with the right measuring 112 mm and the left 118 mm in length; it bears a proximally expansive deltopectoral crest that is taller than the ulnar crest, representing another autapomorphy.^[9] The metacarpus is relatively short, comprising less than 80% of humerus length, with metacarpals I–III at least 11 mm each and the robust fourth metacarpal estimated at 40–75 mm (left preserved at 38 mm long and 9–14 mm wide).^[9] The wing finger phalanges are elongated, including the first at ~109 mm, the second at 135 mm, and the third at 129 mm on the right side, with manual unguals measuring 22 mm (digit I), 16 mm (digit II), and 15 mm (digit III). The pelvic region and hindlimbs exhibit adaptations for support. The sacrum includes five vertebrae with limited fusion, featuring two oval fenestrations but poor preservation that obscures clear articulations.^[9] The hindlimb bones are robust, particularly the fourth metatarsal, which has a diameter 2.5 times that of metatarsals I–III (3 mm versus 1.2–1.6 mm), an autapomorphy emphasizing its strength.^[9] Pedal unguals are recurved, with digit I showing 114° curvature, aiding in grasping.

Functional morphology

Flight and aerial capabilities

Dearc's flight capabilities are inferred from its postcranial skeleton, which exhibits adaptations for powered aerial locomotion typical of early pterosaurs. The specimen, representing a juvenile or subadult individual, possessed a wingspan estimated at over 2.5 meters, potentially exceeding 3 meters, making it one of the largest known Jurassic pterosaurs and indicating substantial aerial performance despite its ontogenetic stage.^[9] Pneumatic bones, including a slender humerus measuring 112–118 mm in length with thin walls, contributed to a lightweight build essential for flight efficiency. The fused scapula-coracoid complex and robust sternum further supported the attachment of primary flight muscles, enabling effective wing movement.^[9] Key wing musculature in Dearc included the powerful M. pectoralis, which originated from a large, lobate deltopectoral crest on the humerus—taller and more expansive than the ulnar crest—and inserted along the sternum's heart-shaped plate with its anteriorly deflected margin, facilitating robust downstroke power through humeral adduction and protraction. The M. coracobrachialis attached via an oval depression on the proximal humerus, aiding in humeral protraction and wing pronation during the flight cycle. These muscle scars and bony robusticity, with a humeral diaphysis width of 9–20 mm, suggest a strong yet efficient system for generating lift, distinct from the more limited capabilities inferred in smaller basal pterosaurs like Rhamphorhynchus. Aerodynamic features of Dearc's wings likely included an elongated fourth metacarpal and extended phalanges, such as the first wing phalanx measuring 109 mm, supporting a high aspect ratio for enhanced glide efficiency and possible membrane attachments along the trailing edge. Well-developed humeral crests indicate adaptations for sustained flapping, while the overall wing morphology points to a flapping-gliding style, potentially incorporating dynamic soaring in coastal environments.^[9] An elongate skull with an inclined quadrate may have reduced drag during flight, converging on later pterodactyloid designs.^[9] These traits underscore the early evolution of large-body flight in non-pterodactyloid pterosaurs during the Middle Jurassic, predating the Cretaceous giants and challenging previous assumptions of size-limited aerial performance in Jurassic forms.^[9] Dearc's anatomy suggests that deformable wing structures capable of high-performance flight originated earlier than previously thought, enabling exploitation of diverse aerial niches.

Terrestrial behavior and diet

Dearc sgiathanach exhibited scansorial and potentially arboreal behaviors, inferred from the strongly recurved manual and pedal unguals that facilitated climbing on trees or rocks. The manual unguals displayed curvatures of 114° for digit I, 106° for digit II, and 99° for digit III, while the pedal unguals measured 117° for digit I, 111° for digit II, and 103° for digit III; these values align with those observed in perching and climbing birds such as psittaciforms, piciforms, and passeriforms, indicating strong gripping capabilities.^[11] Well-developed flexor processes and sesamoids on the unguals further supported precise grasping during ascent or perching.^[11] Robust forelimbs, evidenced by a humerus length of 112–118 mm with a prominent deltopectoral crest, likely enabled launches from elevated perches, enhancing ambush strategies in varied terrains.^[11] On the ground, Dearc likely moved quadrupedally, with its claws capable of retraction to avoid snagging during terrestrial locomotion.^[11] A relatively short neck, comprising six postaxial cervical vertebrae totaling approximately 135 mm in length, combined with a long tail of about 190 mm spanning ten caudal vertebrae, provided balance and stability during such movements.^[11] The tail's elongation, typical of non-pterodactyloid pterosaurs, would have counteracted shifts in body mass, particularly when navigating uneven coastal substrates.^[11] Dearc's diet was primarily piscivorous or carnivorous, suited to capturing hard-shelled prey such as fish or crustaceans in nearshore environments. Conical anterior fangs and prism-shaped posterior teeth, with observed wear including chipped apices and enamel flaking, indicate adaptation for piercing and crushing tougher foodstuffs.^[11] Weak temporal jaw adductors were likely compensated by robust anterior palatal musculature, particularly the M. pterygoideus dorsalis, allowing effective handling of resistant prey through ventral compression rather than lateral biting force.^[11] This configuration suggests opportunistic feeding, where Dearc could exploit a range of available resources without specialized jaw power.^[11] The species probably utilized coastal lagoon habitats for foraging, leveraging arboreal perches along shorelines for ambush predation on aquatic or semi-aquatic prey.^[11] Such environments, characteristic of the Middle Jurassic Bathonian deposits on the Isle of Skye, would have offered proximity to both terrestrial perches and water bodies teeming with suitable quarry.^[11]

Classification and paleoecology

Phylogenetic position

Dearc sgiathanach is classified within basal Pterosauria, specifically in the clade Rhamphorhynchidae under the subfamily Rhamphorhynchinae and the tribe Angustinaripterini.^[9] This placement positions it among long-tailed, non-monofenestratan pterosaurs, characterized by a robust build and features transitional toward later pterodactyloid forms.^[9] Phylogenetic analyses using a matrix of 58 taxa and 155 characters recovered Dearc in a subclade with the Chinese genera Angustinaripterus longicephalus and Sericipterus wucaiwanensis, forming the monophyletic Angustinaripterini with 491 parsimony steps and a consistency index of 0.440.^[9] Additional clade diagnostics encompass a low and elongate skull (height-to-length ratio <0.2), a large antorbital fenestra comprising 20%–35% of skull length and exceeding 80% of orbit height, an inclined quadrate at 130°–140°, and a slender humeral diaphysis bearing a prominent muscle scar tubercle.^[9] Dearc exhibits autapomorphies such as tri-tubular vomers meeting in a trident-shaped contact, a pre-choanal depression on the palatal surface of the maxilla, enlarged optic lobes, and a robust fourth metatarsal approximately 2.5 times the diameter of metatarsals I–III.^[9] Transitional traits, including elongate cranial proportions and the presence of exapophyses on cervical vertebrae, suggest convergent evolution with pterodactyloids, potentially linked to achieving large body size earlier in pterosaur evolution.^[11] Subsequent phylogenetic updates, including analyses incorporating new Middle Jurassic specimens and a detailed 2025 osteological study, have confirmed Dearc's position within Angustinaripterini, emphasizing its role as a sister taxon to Angustinaripterus and Sericipterus.^[11]^[2] It differs from the related rhamphorhynchid Dorygnathus in greater skeletal robusticity, particularly in the humerus and pedal elements, supporting its distinct placement in a subclade of larger-bodied forms rather than basal rhamphorhynchines.^[9] This topology underscores Dearc's basal position among non-monofenestratans while highlighting derived features that bridge early and later pterosaur diversification.^[11]

Evolutionary and ecological implications

The discovery of Dearc sgiathanach provides critical evidence that large-bodied pterosaurs, with wingspans exceeding 2.5 m, evolved much earlier in the Jurassic period than previously recognized, pushing back the timeline for significant size increases in non-pterodactyloid lineages.^[1] This challenges the long-held view of a size constraint limiting Jurassic pterosaurs to wingspans of around 1.6–1.8 m, as Dearc's juvenile status—confirmed by bone histology showing active growth—suggests its adult form could have reached over 3 m.^[1] Such findings indicate that evolutionary pressures for gigantism in flying reptiles were already active by the Middle Jurassic Bathonian stage, approximately 168 million years ago.^[2] Dearc exhibits a mosaic of primitive and derived traits, including exapophyses on its cervical vertebrae—features typically associated with later pterodactyloid pterosaurs—that likely evolved convergently to support larger body masses and enhanced neck flexibility.^[2] This transitional morphology highlights a pattern of modular evolution in pterosaur lineages, where advanced cranial elements (such as an elongate skull and inclined quadrate) coexisted with more basal neuroanatomy, bridging early and later forms without a strict linear progression toward pterodactyloids.^[1] Phylogenetic analyses position Dearc within the Angustinaripterini clade, underscoring how such convergences across non-monofenestratan groups contributed to diverse adaptive radiations during the Jurassic.^[1] Ecologically, Dearc occupied the role of an apex aerial predator in the lagoonal and marginal marine habitats of the Middle Jurassic Hebridean archipelago, preying on fish and possibly harder-shelled prey as inferred from its dentition of elongate fangs and conical peg teeth with wear patterns.^[1] Its large size and flight adaptations allowed it to dominate aerial niches that would later be filled by early birds, while coexisting with smaller terrestrial predators such as theropods and crocodylomorphs in a dynamic coastal ecosystem.^[2] This positioning suggests pterosaurs maintained ecological versatility in island-like settings, influencing food web structures before the diversification of avian competitors. The Skye specimens, including Dearc from the Lealt Shale Formation and the contemporaneous Ceoptera evansae from the Kilmaluag Formation, underscore biases in the Middle Jurassic fossil record, where taphonomic factors have historically underestimated pterosaur diversity and abundance.^[12] These finds indicate a previously unrecognized burst of morphological and size variation, implying that Scottish marine deposits hold untapped potential for further discoveries that could further illuminate this "pterosaur gap."^[4]