Brancasaurus is a genus of plesiosaur that lived during the Berriasian stage of the Early Cretaceous period in what is now Germany. The only known species is B. brancai, first described in 1914 from a relatively complete fossil skeleton discovered four years earlier near Gronau, North Rhine-Westphalia. The scientific name of the taxon honours the German paleontologist Wilhelm von Branca. The holotype skeleton represents the most complete Early Cretaceous plesiosaur specimen known from Europe. Another, more fragmentary skeleton of the same taxon was assigned to the newly erected genus Gronausaurus in 2013, before being recognized as a junior synonym of Brancasaurus in a revision published three years later. Additional specimens assigned or potentially referable to Brancasaurus have been recovered from the type locality and from Lower Saxony, some of which had previously been referred to the genus Plesiosaurus by Ernst Koken.
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 elasmosaurids, a group to which it was long assigned. It differed, however, by its more reduced size, the probable subadult holotype measuring in length. Since 2010, several phylogenetic analyses have recovered Brancasaurus as a member, or a close relative, of the Leptocleididae. Fossil remains of Brancasaurus originates from deposits who likely represented a large continental freshwater lake.
Discovery and naming
The holotype specimen of Brancasaurus brancai originates from a clay pit located near the city of Gronau, North Rhine-Westphalia in Germany. It 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 1914 described the specimen in detail. He published his work in a commemorative volume on the occasion of the seventieth birthday of Wilhelm von Branca, his former mentor in the field, in which he honored by assigning this scientific name of the new taxon. The described specimen is since stored at the University of Münster, where it is numbered as GPMM A3.B4. It is a fairly complete subadult skeleton, 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 Early 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. The holotype skeleton thus constitutes the most complete known European plesiosaur specimen from the Early Cretaceous. In the 2016 redescription of Brancasaurus, Sven Sachs and colleagues assigned a second, more fragmentary subadult individual to the genus, numbered as GZG.BA.0079 at the University of Göttingen. This specimen 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. Sachs and colleagues also noted that a cervical centra from Jurassic-Cretaceous boundary strata of the Purbeck Group in southern England exhibits a morphology comparable to that of B. brancai, suggesting that the taxon may have occurred outside Germany.
Putative and formal synonyms In a 1887 paper, the German paleontologist Ernst Koken described two species of Plesiosaurus, P. degenhardti and P. limnophilus, based on more or less fragmentary remains discovered in Lower Saxony. The holotype of P. degenhardti consists of a specimen preserving twenty-one dorsal vertebrae and their associated ribs, recovered from Obernkirchen. P. limnophilus, in contrast, was established on the basis of three isolated cervical vertebrae from the localities of Ummeln and Kniggenbrink, with the holotype originating from the former locality. In 1896, Koken referred to P. degenhardti additional twenty-three cervical and pectoral vertebrae from its type locality. In 1905, Koken described additional fossils from the quarry near Gronau, which he likewise referred to the genus Plesiosaurus. Based on this material, he erected the species P. kanzleri on the basis of two dorsal vertebrae, although he suggested that they might instead belong to the pliosaurid Peloneustes because of their morphology. He also referred three posterior cervical vertebrae and three anterior dorsal vertebrae to another species, P. valdensis, which he had previously considered assigning to P. degenhardti. In addition, Koken assigned to the latter species a series of dorsal vertebrae, a sacral vertebra, and fragments of ribs, femora, and the jaw, which he interpreted as belonging to a more complete skeleton that had most likely been destroyed prior to its discovery.
Although the vast majority of the material, as well as the three species described by Koken, are now regarded as nomina dubia, Sachs et al. (2016) considered that most of these fossils show only minor differences from B. brancai. More specifically, they considered the holotype specimens of P. limnophilus and P. kanzleri, as well as the now-lost specimens assigned to P. valdensis and P. degenhardti in 1905, to be indistinguishable from B. brancai. They nevertheless emphasized that theses referrals, as well as the possible synonymy of P. limnophilus and P. kanzleri with B. brancai, cannot be confirmed with certainty due to the absence of a diagnostic combination of characters allowing an unambiguous identification. However, they considered that the original fossils of P. degenhardti from Obernkirchen represent a distinct taxon from B. brancai, due to the morphology of its cervical vertebrae . Sachs and colleagues also suggested that the English taxon Hastanectes valdensis, whose partial holotype skeleton is known from the Valanginian Wadhurst Clay Formation, is of uncertain generic distinction from Brancasaurus.
Two years after the initial discovery of the holotype of B. brancai, a second, more fragmentary skeleton was discovered in the same quarry, approximately higher in the stratigraphic column. Since numbered as GPMM A3.B2, this specimen 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. Initially assigned to B. brancai in 1961, the specimen was designated by the German paleontologist Oliver Hampe as the holotype of a new genus and species, Gronausaurus wegneri, in a study published in 2013. Sachs et al. 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.
Description
Brancasaurus was a medium-sized plesiosaur, with the holotype specimen measuring in length; this specimen likely belongs to a subadult, judging by the unfused sutures in the vertebrae as well as the development of processes on the limbs and pubis.
Skull
The skull of the holotype, which measures long, is long and narrow, with a tapered snout that slopes downwards at an angle of 15°. The eye sockets were roughly the same size as the temporal openings immediately behind them. A narrow, rounded ridge along the middle of the top surface of the skull extends from near the front of the premaxilla to the back of the eye sockets. The frontal bones form a rectangular bar that separates the eye sockets down the middle. A ridge running across the bar intersects with the forward-extending ridge to produce a dagger-shaped protrusion. The jugal bone, which extends from the bottom of the eye socket back to the level of the temporal openings, is entirely bordered on its bottom by the maxilla. The squamosal bones arch around to form the curved back of the skull, and bear a ridge on top for attachment of neck muscles. There is also a ridge at the point where the two bones fuse. A cast of the braincase shows impressions of the semicircular canals and membranous inner ear, as well as canals of the hypoglossal, accessory, glossopharyngeal, and vagus nerves, which can also be observed on the bony exoccipital-opisthotic of the braincase. On the imperfectly-preserved lower jaw, the coronoid eminence seems to be relatively low, judging by the narrow and slightly curved top edge of the surangular bone. While the teeth have been lost, they were initially described as long, slender, and awl-shaped, with rough ridges on the outer surfaces. Although it has been suggested that Brancasaurus had very reduced tooth sockets in the premaxilla, as in Leptocleidus, this is impossible to verify because of damage to this portion of the skull.
Vertebral column
The entire neck bears 37 cervical vertebrae, and is approximately long. The centra of the vertebrae are wider than they are tall or long. Both ends of each vertebra are slightly concave, meaning that the vertebrae are amphicoelous. The sides of the vertebrae are likewise weakly concave; unlike many other long-necked plesiosaurs, they did not bear a ridge on the side (although this may be affected by age). The neural spines of the vertebrae are distinctively shaped like shark fins, being high and triangular. There are three pectoral vertebrae at the neck-body transition, which are weakly concave, taller than they are long, and have rectangular-shaped neural spines that are directed slightly backwards. The cervical and pectoral vertebrae have deep indentations through which the notochord passed.