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| Ophthalmothule cryostea
Roberts, Druckenmiller, Cordonnier, Delsett & Hurum, 2020
Illustration by Esther van Hulsen.
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Abstract
Cryptoclidids are a major clade of plesiosauromorph plesiosaurians best known from the Middle—Late Jurassic, but little is known regarding their turnover into the Early Cretaceous. Of the known cryptoclidid genera, most preserve only a limited amount of cranial material and of these Cryptoclidus eurymerus, displays the most complete, but compressed cranium. Thus, the lack of knowledge of the cranial anatomy of this group may hinder the understanding of phylogenetic interrelationships, which are currently predominantly based on postcranial data. Here we present a nearly complete adult cryptoclidid specimen (PMO 224.248) representing a new genus and species Ophthalmothule cryostea gen et sp. nov., from the latest Jurassic to earliest Cretaceous part of the Slottsmøya Member, of central Spitsbergen. The holotype material preserves a complete cranium, partial mandible, complete and articulated cervical, pectoral and anterior to middle dorsal series, along with the pectoral girdle and anterior humeri. High resolution microcomputed tomography reveals new data on the cranial anatomy of this cryptoclidid, including new internal features of the braincase and palate that are observed in other cryptoclidids. A phylogenetic analysis incorporating new characters reveals a novel tree topology for Cryptoclididae and particularly within the subfamily Colymbosaurinae. These results show that at least two cryptoclidid lineages were present in the Boreal Region during the latest Jurassic at middle to high latitudes.
Systematic Palaeontology
Sauropterygia Owen, 1860
Plesiosauria de Blainville, 1835
Plesiosauroidea Welles, 1934
Cryptoclididae Williston, 1925
Ophthalmothule gen. nov.
Ophthalmothule cryostea sp. nov.
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| Figure 2: Quarry map and reconstruction of Ophthalmothule cryostea, PMO 224.248. (A) Drawing from a combination of field and laboratory drawings in ventral view (modified from Delsett et al. (2016)); (B) skeletal reconstruction of PMO 224.248, where red indicates preserved elements. Scale bar equals 50 cm. Drawn by Aubrey Jane Roberts. |
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| Figure 3: The cranium of Ophthalmothule cryostea, PMO 224.248 in dorsal view. (A) Photo of PMO 224.248, (B) µCT reconstruction and (C) interpretation.
Abbreviations: bo, basioccipital; en, external naris; ex-op, exoccipital-opisthotic; f, frontal; ifv, interfrontal vacuity; mx, maxilla; p, parietal; pif, pineal foramen; pm, premaxilla; pop, paraoccipital process; q, quadrate; so, supraoccipital; sq, squamosal; tfen, temporal fenestra. Scale bar equals 5 cm.
Photograph and reconstruction by Aubrey Jane Roberts.
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Holotype: PMO 224.248
Occurrence: The holotype specimen PMO 224.248 was excavated from the north-facing slopes of Wimanfjellet (Mt. Wiman), from the upper part of the Slottsmøya Member, Agardhfjellet Formation, central Spitsbergen: GPS coordinates UTM 33X E523620 N8696396 (Fig. 1). The specimen was located 38.5 m above the yellow storm deposit marker bed (0 m in log), and is late Volgian (latest Tithonian/ early Berriasian) in age.
Etymology: Ophthalmothule. Ophthalmo, meaning eye. Thule is a term used for the northern-most region of the world. Together they make “North eye”. Species name, cryostea, meaning “frozen bones”.
Differential diagnosis:
A moderately sized cryptoclidid plesiosaur (estimated body length of 5.0–5.5 m), possessing the following autapomorphies unique among Cryptoclididae (*) and unique character combinations: premaxilla bears 6 alveoli (5 in Tricleidus seeleyi and Muraenosaurus leedsii); medial process of premaxilla terminates anterior to the posterior margin of external naris (*); maxilla estimated to contain a similar number of alveoli (>16) as in in Cryptoclidus eurymerus (18) and Tricleidus seeleyi (15); frontal twice as anteroposteriorly long as parietal (subequal or shorter in Cryptoclidus eurymerus and M. leedsii); frontal participates in the medial and posterior margins of the external naris (participates posteriorly in M. leedsii); presence of an interfrontal vacuity (absent in M. leedsii); dorsoventrally low but mediolaterally narrow sagittal crest (flat and mediolaterally broad in Kimmerosaurus langhami); quadrate articulates anterolaterally to the pterygoid (posteromedially in Tricleidus seeleyi and M. leedsii); lateral cotyle of quadrate larger than medial cotyle (reversed in S. larseni); basioccipital tubera mediolaterally broad and dorsoventrally flattened (circular in K. langhami and Cryptoclidus eurymerus);
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| A reconstruction of Ophthalmothule cryostea in its natural environment.
Illustration by Esther van Hulsen.
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Conclusion:
Ophthalmothule cryostea (PMO 224.248) represents the temporally youngest occurrence of a plesiosaurian from the Slottsmøya Member (Agardhfjellet Formation) of central Spitsbergen. Ophthalmothule cryostea represents the fourth genus described from the member, although several other cryptoclidid specimens remain to be described. Ophthalmothule cryostea is one of the few cryptoclidids with detailed cranial osteology available, providing much needed morphological information for understanding the interrelationships of cryptoclidids. In addition, this specimen uniquely preserves a complete cervical series found in articulation, offering future possibilties to test current hypotheses on plesiosaurian neck-flexibility and evolution. As the specimen was found in the section encompassing the Jurassic–Cretaceous boundary, Ophthalmothule cryostea along with the Russian Abyssosaurus nataliae represent the youngest cryptoclidid genera in Boreal and sub-Boreal regions. The phylogenetic results of this study indicate that two separate clades of cryptoclidids were present in the latest Jurassic in the Boreal region of Spitsbergen and the sub-Boreal region of Russia.
Aubrey Jane Roberts, Patrick S. Druckenmiller, Benoit Cordonnier, Lene L. Delsett and Jørn H. Hurum. 2020. A New Plesiosaurian from the Jurassic–Cretaceous Transitional Interval of the Slottsmøya Member (Volgian), with Insights Into the Cranial Anatomy of Cryptoclidids using Computed Tomography. PeerJ. 8:e8652. DOI: 10.7717/peerj.8652
Aubrey Jane Roberts gravde frem en sensasjon fra fjellet på Svalbard nrk.no/viten/xl/1.14921060
