Brancasaurus (meaning "Branca's lizard") is a genus of plesiosaur which lived in a freshwater lake in the Early Cretaceous of what is now North Rhine-Westphalia, Germany. With a long neck possessing vertebrae bearing distinctively-shaped "shark fin"-shaped neural spines, and a relatively small and pointed head, Brancasaurus is superficially similar to Elasmosaurus, albeit smaller in size at in length as a subadult.
The type species of this genus is Brancasaurus brancai, first named by Theodor Wegner in 1914 in honor of German paleontologist Wilhelm von Branca. Another plesiosaur named from the same region, Gronausaurus wegneri, most likely represents a synonym of this genus. While traditionally considered as a basal member of the Elasmosauridae, Brancasaurus has more recently been recovered as a member, or close relative, of the Leptocleididae, a group containing many other freshwater plesiosaurs.
Discovery and naming
The holotype specimen of Brancasaurus brancai is GPMM A3.B4, stored at the University of Münster. It originates from a clay pit near the city of Gronau, North Rhine-Westphalia in Germany. The specimen was discovered in July 1910 by workers in the clay pit, who dug it out using pickaxes; in doing so, they damaged the specimen (in particular, the pubis had been broken into 176 pieces), and left behind a number of small fragments that were later personally collected by paleontologist Theodor Wegner, who in 1928 described the specimen in detail. The skeleton is fairly complete, consisting of various parts of the skull, most of the vertebrae, several isolated ribs and gastralia, parts of the pectoral and pelvic girdles, both humeri, one femur, and various foot bones from the flippers. Over time, a number of parts have been lost, including several pieces of the skull, teeth, gastralia and caudal vertebrae, a second femur, and a radius, tibia, and fibula. A wax endocast of the brain of the type specimen is stored as SMF R4076 in the Naturmuseum Senckenberg.
The clay pit from which the type specimen originates is part of the Isterberg Formation in the Bückeberg Group, also known in the past as the "German Wealden facies". The Bückeberg Group, which is divided into six zones, belongs to the Berriasian of the Cretaceous, with the boundary between the Berriasian and the Valanginian being at the top of the group. The parts of the Isterberg Formation exposed at Gronau belong to the zones "Wealden 5" and "Wealden 6", which correspond to the uppermost-Berriasian. A second, more fragmentary subadult individual, GZG.BA.0079, consists of the pubis, ischium, and several vertebral components; it originates from the slightly lower Deister Formation ("Wealden 3") in the Bückeberg Group, and can only be referred to Brancasaurus sp., since it is relatively incomplete and differs in several minor vertebral characteristics from the type of B. brancai. Other probable but isolated Brancasaurus elements come from outcrops of the Isterberg and Fuhse Formations in Lower Saxony; the latter formation is also in the Bückeberg Group.
Synonyms
The specimen GPMM A3.B2 consists of teeth, parts of the jaws, the braincase and other fragmentary parts of the skull, vertebrae, pieces of ribs, part of the pectoral girdle, the entire pelvic girdle, one complete and one partial humerus, an ulna, two femora, a fibula, and various foot bones. While this specimen was originally assigned to Brancasaurus, Hampe (2013) referred it to a new genus and species, Gronausaurus wegneri. It was discovered some higher in the stratigraphic column than the type specimen of Brancasaurus. Later analysis found that this specimen, which was mature, was virtually indistinguishable from the type of Brancasaurus with the exception of the length of the ischium, the height of the cervical neural spines, the width of the cervical centra, and whether the dorsal neural spines are constricted at their base. These minor differences can probably be attributed to either individual-based or age-based variation, supporting G. wegneri as a junior synonym of B. brancai.
E. Koken named Plesiosaurus limnophilus in 1887 based on isolated cervical vertebrae from outcrops of the Bückeberg Group in Lower Saxony. From the same locality, Koken subsequently named two further species of Plesiosaurus, P. degenhardti and P. kanzleri, and also referred some material to P. valdensis. All of this material is not particularly diagnostic, and has been partially lost; thus, they have been considered nomina dubia. Sachs et al. considered all of these to represent remains of Brancasaurus, with the exception of P. degenhardti, which was retained as a nomen dubium on account of lacking the distinctive cervical neural spines of Brancasaurus.