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Reassessment of 'Plesiosaurus' megacephalus (Sauropterygia: Plesiosauria) from the Triassic-Jurassic boundary, UK

Adam S. Smith

Plain Language Abstract

Plesiosaurians, also called plesiosaurs, are an extinct group of Mesozoic marine reptiles with a global fossil record that ranges from the Rhaetian (Latest Triassic) to the Maastrichtian (Latest Cretaceous). The holotype specimen of one plesiosaurian skeleton, 'Plesiosaurus' megacephalus Stutchbury, 1846 (BRSMG Cb 2335) from the Triassic-Jurassic boundary of Street-on-the-Fosse, Somerset, UK, was destroyed during an air raid on Bristol during WWII. However, plaster casts of the skull and right forelimb were produced prior to its destruction. Together with historical photographs and original descriptions, these casts provide three-dimensional anatomical data for a modern systematic diagnosis of the taxon. The taxon is compared with other Lower Jurassic plesiosaurians demonstrating that 'Plesiosaurus' megacephalus is distinct and so a new genus is erected to accommodate it: Atychodracon gen. nov. As it is the type species of the genus, clarification and description of Atychodracon megacephalus is the first step towards understanding the diversity and resolving the systematics of closely related large-bodied plesiosaurians from the earliest Jurassic.

Resumen en Español

Revaloración de 'Plesiosaurus' megacephalus (Sauropterygia: Plesiosauria) del límite Triásico-Jurásico, Reino Unido

El holotipo de 'Plesiosaurus' megacephalus Stutchbury, 1846 (BRSMG Cb 2335) fue destruido en un ataque aéreo a Bristol durante la Segunda Guerra Mundial. El espécimen comprendía un esqueleto completo proveniente del límite Triásico-Jurásico de Street-on-the-Fosse, Somerset, Reino Unido. Moldes de yeso del cráneo y del miembro anterior derecho fabricados previamente a la destruccion del holotipo, junto con fotografías históricas y descripciones originales, proveen información anatómica tridimensional para una diagnosis sistemática moderna del taxón. Un neotipo previamente propuesto (LEICS G221.1851) es por ende considerado como espécimen de referencia. Una comparación detallada con otros plesiosaurios del Jurásico Inferior demuestra que 'Plesiosaurus' megacephalus es un taxón diferenciable, distinto de Rhomaleosaurus sensu stricto y Eurycleidus arcuatus, y en consecuencia se erige un nuevo género para el mismo: Atychodracon gen. nov. La clarificación y descripción de Atychodracon megacephalus es un paso vital hacia la resolución de la sistemática y la comprensión de la diversidad de los plesiosaurios de gran tamaño del límite Triásico-Jurásico.

Palabras clave: género nuevo; Plesiosauria; Triásico-Jurásico; escaneo láser 3D; molde de yeso; Rhomaleosauridae

Traducción: Diana Elizabeth Fernández

Résumé en Français

Réévaluation de 'Plesiosaurus' megacephalus (Sauropterygia: Plesiosauria) de la limite Trias-Jurassique, Royaume-Uni

Le spécimen holotype de 'Plesiosaurus' megacephalus Stutchbury, 1846 (BRSMG Cb 2335) a été détruit lors d'un raid aérien sur Bristol lors de la Seconde Guerre mondiale. Le spécimen comprenait un squelette complet de la limite Trias-Jurassique de Street-on-the-Fosse, Somerset, Royaume-Uni. Des plâtres du crâne et de la patte antérieure droite produits avant sa destruction, en conjonction avec des photographies historiques et descriptions originales, fournissent des données anatomiques en trois dimensions pour un diagnostic systématique moderne du taxon. Un spécimen néotype précédemment proposé (Leics G221.1851) est donc considéré comme un spécimen référé. Une comparaison détaillée avec d'autres Plésiosauriens du Jurassique inférieur démontre que 'Plesiosaurus' megacephalus est un taxon distincte, séparée de Rhomaleosaurus sensu stricto et Eurycleidus arcuatus, et donc un nouveau genre est érigé pour l'accueillir: Atychodracon gen. nov. La clarification et la description de Atychodracon megacephalus est une étape essentielle vers la compréhension de la diversité et afin de résoudre la systématique des grands-corps Plésiosauriens de la limite Trias-Jurassique.

Mots-clés: nouveau genre; Plesiosauria; Trias-Jurassique; Scanner laser 3D; plâtre; Rhomaleosauridae

Translator: Kenny J. Travouillon

Deutsche Zusammenfassung

Neubewertung von 'Plesiosaurus' megacephalus (Sauropterygia: Plesiosauria) aus der Trias-Jura Grenze, Großbritannien

Der Holotypus von 'Plesiosaurus' megacephalus Stutchbury, 1846 (BRSMG Cb 2335) wurde während eines Bombenangriffs auf Bristol im zweiten Weltkrieg zerstört. Das Stück bestand aus einem kompletten Skelett aus der Trias-Jura Grenze von Street-on-the-Fosse, Somerset, Großbritannien. Abgüsse des Schädels und des rechten Vorderbeins, die vor der Zerstörung angefertigt worden waren, stellen zusammen mit historischen Fotos und Originalbeschreibungen dreidimensionale anatomische Daten für eine moderne systematische Diagnose des Taxons zur Verfügung. Ein früher vorgeschlagener Neotypus (LEICS G221.1851) wird daher als Referenzstück angesehen. Ein detaillierter Vergleich mit anderen unterjurassischen Plesiosauriern zeigt, dass 'Plesiosaurus' megacephalus ein unterscheidbares Taxon ist, getrennt von Rhomaleosaurus sensu stricto und Eurycleidus arcuatus. Daher wird eine neue Gattung aufgestellt: Atychodracon gen. nov. Die Abklärung und Beschreibung von Atychodracon megacephalus ist ein wichtiger Schritt zum Verständnis der Diversität und zur Lösung der Systematik dieser großen Plesiosaurier aus der Trias-Jura Grenze.

Schlüsselwörter: neue Gattung; Plesiosauria; Trias-Jura; 3D Laserscan; Abguss; Rhomaleosauridae

Translator: Eva Gebauer

Arabic

Translator: Ashraf M.T. Elewa

 

 

FIGURE 1. Historical photograph of the holotype skeleton (BRSMG Cb 2335) of Atychodracon megacephalus (Stutchbury, 1846). Photograph taken from glass plate negative in the Bristol City Museum & Art Gallery, originally published by Swinton (1948). Bristol City Museum & Art Gallery, reproduced with permission. Length of skeleton equals 4960 mm.

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FIGURE 2. Plaster cast (BGS GSM 118410) of the holotype (BRSMG Cb 2335) skull of Atychodracon megacephalus (Stutchbury, 1846) in ventral (palatal) view. Three dimensional scan with texture (colour) removed. Scale bar equals 100 mm.

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FIGURE 3. Plaster cast (BGS GSM 118410) of the anterior part of the holotype (BRSMG Cb 2335) skull of Atychodracon megacephalus (Stutchbury, 1846). 1-3, dorsal views, 4-6, ventral views. 1 and 4, three dimensional scans with texture (colour) removed, 2 and 5, photographs, 3 and 6, interpretations. Abbreviations: aiv - anterior interpterygoid vacuity; den - dentary; en - external naris; fo - nutritive foramina; fr - frontal; in - internal naris; mx - maxilla; pal - palatine; pmx - premaxilla; pt - pterygoid; sp - splenial; t - teeth; vom - vomer. Cross-hatching represents original broken surfaces, stippling represents original matrix. Scale bar equals 100 mm.

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FIGURE 4. Plaster cast (BGS GSM 118410) of the anterior portion of the holotype (BRSMG Cb 2335) skull of Atychodracon megacephalus (Stutchbury, 1846). 1-3, right lateral view, 4-6, left lateral view, 7-9, oblique posterolateral view. 1, 4, 7, three dimensional scans with texture (colour) removed, 2, 5, 8, photographs, 3, 6, 9, interpretations. Abbreviations: aiv - anterior interpterygoid vacuity; ch - 'chin'; cor - coronoid; den - dentary; en - external naris; in - internal naris; mx - maxilla; pal - palatine; pmx - premaxilla; pt - pterygoid; sp - splenial; t - teeth; vom - vomer. Cross-hatching represents original broken surfaces, stippling represents original matrix. Scale bar equals 100 mm.

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FIGURE 5. Posterior part of a plaster cast (BGS GSM 118410) of the holotype (BRSMG Cb 2335) skull of Atychodracon megacephalus (Stutchbury, 1846) in ventral (palatal) view. 1, three dimensional scan with texture (colour) removed, 2, photograph, 3, interpretation. Abbreviations: ang - angular; ar - articular; at - atlas; ax - axis; bs/bo? - basisphenoid or basioccipital; c3 - cervical vertebra 3; c8 - cervical vertebra 8; cr - cervical ribs; ec? - probable ectopterygoid; hy - hyoid; imf - internal mandibular foramen; mb - medial boss of retroarticular process; piv - posterior interpterygoid vacuity; ppt - posterior process of pterygoid; pro - possible additional process between zygapophyses; ps - parasphenoid; pt - pterygoid; ? - unknown element. Cross-hatching represents original broken surfaces, stippling represents original matrix. Scale bar equals 100 mm.

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FIGURE 6. Transverse cross-sections of the plaster cast (BGS GSM 118410) of the holotype (BRSMG Cb 2335) skull of Atychodracon megacephalus (Stutchbury, 1846), showing surface morphology. 1, ventral view of skull showing position of cross-sections, 2-7, cross-sections, dorsal surface towards top. Abbreviations: aiv - anterior interpterygoid vacuity; chan - channels anterior to external nares; fl - ventrolaterally directed flanges of pterygoid; hy - hyoid; in - external nares; k - ventral keel on parasphenoid; piv - posterior interpterygoid vacuity; vlc - ventral longitudinal crest; vlf - ventrolateral flange. Scale bar equals 100 mm (in 1) and 50 mm (in 2-7 ), note that cross-sections are to scale relative to each other.

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FIGURE 7. Plaster cast (BGS GSM 118410) of the ventral surface of the right forelimb of the holotype of Atychodracon megacephalus (Stutchbury, 1846) (BRSMG Cb 2335). 1, three dimensional scan with texture (colour) removed, 2, photograph, 3, interpretation. Abbreviations: 1 - distal carpal 1; 2-3 - fused distal carpals 2 and 3; 4 - distal carpal 4; h - humerus; i - metacarpal i; ii - metacarpal ii; iii - metacarpal iii; iv - metacarpal iv; v - metacarpal v; in - intermedium; r - radius; radiale; ra - radiale; u - ulna; ul - ulnare. Scale bar equals 100 mm.

figure7

 

TABLE 1. Selected measurements (in mm) and vertebral counts for BRSMG Cb 2335 as given by Stutchbury (1846, measurements converted from inches), Sollas (1881, measurements converted from inches) and Swinton (1948), and measurements of BGS GSM 118410 for comparison.

 

MeasurementStutchbury (1846)Sollas (1881)Swinton (1948)BGS GSM 118410
Total length 4953 4953 4960 n/a
Neck length n/a 1430 1350/1370 (two conflicting measurements are given) n/a
Cervical vertebrae count 29 29-30 30 n/a
Tail length n/a 1384 1300 n/a
Caudal vertebrae count 31 n/a 32 n/a
Skull length (from anterior tip of snout to the...) 813 (... end of the articular bones) 762 (...back of the quadrate) 830 (...back of the angular) 830 (...back of the angular)
Premaxilla tip to external nares length 244 n/a n/a 235
Premaxilla tip to orbit anterior margin length 292 n/a n/a 290
Premaxillary rostrum length n/a n/a n/a 110
Premaxillary rostrum width 133 n/a n/a 110
External naris length 28 n/a n/a 27.6
External naris width 12.7 n/a n/a 11.2
Anterior interpterygoid vacuity length n/a n/a n/a approx. 60
Mandibular symphysis length 142 n/a 150 127.1
Maximum width of mandible adjacent to mandibular symphysis n/a n/a n/a 128.5
Maximum skull width (across mandible) n/a n/a n/a 455
Skull width across angulars n/a n/a 340 340
Length of premaxilla to basisphenoid n/a n/a 570 n/a
Length of parasphenoid bar between posterior interpterygoid vacuities n/a n/a 48 n/a
Posterior interpterygoid vacuity length n/a n/a 46 42
Posterior interpterygoid vacuity width n/a n/a 25 24
Maximum width between the outer borders of the posterior interpterygoid vacuities n/a n/a 68 62
Preserved basisphenoid length n/a n/a 36 n/a
Width of mandibular ramus at glenoid n/a n/a n/a 70 (left), 87 (right)
Maximum width of retroarticular process n/a n/a n/a 39
Hyoid length 145 n/a 147 147
Atlas-axis length n/a n/a n/a 98
Entire forelimb length n/a 838 n/a 825
Humerus length 348 348 350 350
Humerus distal width 178 n/a 180 183
Width of humeral head 84 n/a 85 93
Radius length 117 117 113 117
Proximal width of radius 89 n/a 91 91
Distal width of radius 69 n/a 70 69
Ulna length 109 109 105 110
Width of the ulna at midlength 84 n/a 84 81
Femur length n/a 343 340 n/a
Tibia length 114 114 118 n/a
Fibula length 114 114 118 n/a
 

smithAdam S. Smith
Nottingham Natural History Museum
Wollaton Hall
Nottingham, NG8 2AE
UK
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Dr Adam S. Smith is Collections Access Officer (Natural History) at the Nottingham Natural History Museum, Wollaton Hall, UK. His research focuses on the anatomy and evolution of Mesozoic marine reptiles, particularly plesiosaurs and ichthyosaurs. He completed an MSc in palaeobiology at the University of Bristol in 2003 and a PhD at the National University of Ireland in 2007.