neotype. The species had a very unstable taxonomic history, being referred to four different genera by various authors until a new genus name was created for it in 2015. Apart from the destroyed holotype and its three partial casts (that survived), a neotype and two additional individuals are currently referred to Atychodracon megacephalus, making it a relatively well represented rhomaleosaurid.[1]
History of discovery
The type species of Atychodracon was first described and named by
Thaumatosaurus which was regarded by him as a replacement to Seeley's Rhomaleosaurus - creating the new combination T. megacephalus.[1][3]
The
million years ago. However, it is possible that some of the specimens from the area are from slightly younger deposits.[1]
BRSMG Cb 2335 was destroyed during a World War II air raid on 24 November 1940, however detailed descriptions and illustrations of the specimen as well as high quality historical photographs still exist to this day. Additionally, at least three casts are known, including: NHMUK R1309/1310 housed at the Natural History Museum, London, TCD.47762a+b at the Geology Museum, Trinity College Dublin, and BGS GSM 118410 at British Geological Survey, Keyworth, Nottingham. Each of the casts is a replica of parts of the original specimen, and comprises a representation of the skull, nine front neck vertebrae including the atlas-axis, and the right forelimb. In June 2014, three-dimensional digital models of BGS GSM 118410 were produced.[1]
Cruickshank (1994a) described LEICS G221.1851 as a
Barrow-upon-Soar, of Leicestershire, England. It was collected by William Lee during the early 1850s, from the Psiloceras planorbis Zone of Bottom Floor Limestone Member of Blue Lias, 2 meters above the local Rhaetian, thus dating to the early Hettangian. However, a 2015 revision of "P." megacephalus pointed out that according to International Code of Zoological Nomenclature article 51.1, designating a neotype in this case is not required since representations of the holotype exist, and are enough to define the species objectively. Thus, currently LEICS G221.1851 is treated as a specimen referred to Atychodracon megacephalus and not its type material.[1]
Two additional specimens, both housed at the
phylogenetic analysis, WARMS 10875 seems to represent a new unnamed rhomaleosaurid species potentially related to Atychodracon and Eurycleidus.[1][4]
Description
Restoration
Atychodracon is a medium size carnivorous
lower jaw are bowed from the mid to the sides. The mandibular symphysial region is expanded to the sides, nearly equal in width and length. The bottom surface of the dentary close to the mandibular symphysis, to which the splenials contribute on the mid bottom side, shows diverging bars and a midline longitudinal crest. An arrow cleft is present on midline between the dentaries and a large lingual foramen is present on the lower jaw. A medial boss is present on the retroarticular projection.[1]
Atychodracon, based solely on the holotype, has 29-30
tail vertebrae, a projection on the front surface of the cervical ribs, a straight preaxial humerus margin, poorly defined radius and ulna facets on the humerus, a slightly shorter humerus than femur, a shorter ulna than radius, and a tibia and fibula equal length.[1]
Among other
sister taxon of Atychodracon when both are included, which seems to imply a close kinship between the taxa, as originally suggested by Andrews (1922). Yet, this is not supported by all analyses, and despite the difficulty in directly comparing the two, several differences exist. The holotype of Eurycleidus lacks a skull, and the previously referable OUM J.28585 probably represents a new taxon, so little overlapping material exists between the holotypes of Atychodracon and Eurycleidus. However the following differences are notable: in Eurycleidus the midline cleft on the bottom surface of the mandibular symphysis is not bordered by the splenials from the back, like in Atychodracon. In Eurycleidus, an additional large asymmetrical cleft separates the splenials on the midline. Unlike the straight preaxial margin of the humerus of Atychodracon, it is concave in Eurycleidus. Additionally, Atychodracon shows a more stout and robust humerus, and a reverse relation in the radius to ulna lengths (the former being shorter than the letter in Eurycleidus). These distinctions suggest that while Atychodracon is fairly closely related to Eurycleidus, it represents a separate genus.[1]
Classification
In a revision of many pliosauroid taxa, Andrews (1922) was the first to recognize that "P." megacephalus is morphologically more closely related to "Plesiosaurus" arcuatus than to species of the Rhomaleosaurus/Thaumatosaurus complex. He concluded that the two species belong to the same genus, which he erected as Eurycleidus, with the type species being Eurycleidus arcuatus, and E. megacephalus as a referred species. Nevertheless, this was not followed by all authors, such Swinton (1930) who used T. megacephalus and later in 1948 P. megacephalus, to avoid confusion soon after the holotype was destroyed. This conservative name, "P." megacephalus, was followed by Taylor and Cruickshank (1989) and Taylor (1994). However, in 1994 Cruickshank designated a neotype for the species, and due to the Rhomaleosaurus/Thaumatosaurus issue being resolved in favor of the former (while Thaumatosaurus is a nomen dubium) he referred to it by the new combination, Rhomaleosaurus megacephalus.[1][3]
Adam S. Smith in his 2007 thesis on the anatomy and classification of the family
phylogenetic analysis of all valid Rhomaleosaurus species, and most valid rhomaleosaurids. The cladogram below follows the preliminary phylogenetic analysis of Smith & Dyke (2008), with the asterisk noting species recently removed from Rhomaleosaurus to their own genera.[4]
LEICS G221.1851 (neotype) → referred to Atychodracon megacephalus*
NMING F8749 → referred to Atychodracon megacephalus*
NMING F10194 → referred to Atychodracon megacephalus*
Eurycleidus arcuatus
Sthenarosaurus dawkinsi
Meyerasaurus victor
*
Maresaurus coccai
Rhomaleosaurus propinquus
Rhomaleosaurus zetlandicus
Rhomaleosaurus cramptoni
Rhomaleosaurus thorntoni
generic name is derived from Greek ατυχής, atychis, meaning "unfortunate" in reference to the unfortunate destruction of the holotype during a World War II air bombing in 1940, plus δράκωνe, drakon, meaning "dragon" - a common suffix in genus names of various mesozoic reptile groups.[1]
Atychodracon has been included on many occasions in various
phylogenetic analyses, usually as "Rhomaleosaurus" megacephalus. In these analyses, the referred LEICS G221.1851 was used to represent the species due to its higher completeness and being the proposed neotype. The referral of this LEICS G221.1851 to Atychodracon is relatively strong, meaning that this should not affect Atychodracon position in the topology.[1] The following two cladograms are simplified after two recent analyses, showing only the relationships within Rhomaleosauridae, and a few basal
taxa whose position within the family is highly uncertain.
