Knoetschkesuchus is a genus of small atoposaurid crocodylomorph from the Late Jurassic of Germany and Portugal. Two species are known: the German species K. langenbergensis, described by Schwarz and colleagues in 2017 based on two partial skeletons and various isolated bones; and the Portuguese species K. guimarotae, named from over 400 specimens including several partial skeletons. Knoetschkesuchus was a small and short-snouted crocodilian, measuring about in length, that primarily fed on small prey, including invertebrates, amphibians, and mammals. This specialization towards small prey ecologically separated Knoetschkesuchus from most of the other diverse crocodilians that it lived with in the island ecosystem of Jurassic Europe.
Both species were formerly recognized as belonging to Theriosuchus; K. guimarotae was initially named as T. guimarotae, and specimens of K. langenbergensis were initially referred to T. pusillus upon their discovery. Schwarz and colleagues recognized a number of characteristics that united these two taxa to the exclusion of other species of Theriosuchus; in particular, Knoetschkesuchus only has two distinct types of teeth, lacking the leaf-shaped teeth seen in other atoposaurids. Other distinguishing traits include the relatively wide skull, and the presence of the antorbital and mandibular fenestrae in all life stages.
Discovery and naming K. langenbergensis
The material of the type species of Knoetschkesuchus, K. langenbergensis, comes from a marly limestone bed (numbered as Bed 83, not Bed 93 as reported by some publications) containing recrystallized micritic intraclasts, located within the Langenberg Quarry in the Harz Mountains near Goslar, Lower Saxony, Germany. These deposits have been dated to the Upper Kimmeridgian stage of the Jurassic—specifically, to the regional equivalent of the Upper Kimmeridgian known as the Mittleres Kimmeridge, and belong to the Süntel Formation. Although well-preserved, fossils from this quarry were recovered generally by regular blasting operations in the quarry.
Specimens belonging to Knoetschkesuchus are stored at the Dinosaurier-Freilichtmuseum Münchehagen (hereafter denoted by DFMMh/FV) in Rehburg-Loccum, Germany. They are: the type specimen DFMMh/FV 200, the partial skeleton of an adult with a skull; DFMMh/FV 605, the complete skull of a juvenile; DFMMh/FV 261, an isolated angular; DFMMh/FV 790.12, an isolated left dentary; DFMMh/FV 279, an isolated femur; DFMMh/FV 790.11, an isolated metatarsal; and DFMMh/FV 325, a partial skeleton including osteoderms, vertebrae, and ribs. Stereomicroscopy was used to examine the specimens, which were described in a 2017 paper published by Daniela Schwarz, Maik Raddatz, and Oliver Wings.
The genus name Knoetschkesuchus combines the family name of Nils Knötschke, a researcher at the DFMMh who was responsible for the collection, curation, and preparation of Langenberg Quarry specimens, with the common crocodilian suffix suchus, from the Greek souchos ("crocodile"). Meanwhile, the species name langenbergensis is in reference to the provenance of this species from the Langenberg Quarry.
K. guimarotae Schwarz et al. also assigned an additional species to Knoetschkesuchus, K. guimarotae, which was originally classified as a species of Theriosuchus. As reflected by the specific name, K. guimarotae originates from the lignite layers of the Guimarota quarry, located near Leiria, Portugal. There are two primary vertebrate-bearing lignite strata within the so-called Guimarota-strata, which are separated by marly limestone and respectively known as the "Fundschichten" and "Ruafolge" subunits; K. guimarotae is known from both of these layers. They have been assigned to the Alcobaça Formation, a subunit of the Abadia Formation, which has been dated to the Kimmeridgian on the basis of fossil ostracods.
The remains of K. guimarotae are stored at the Institute of Geological Sciences of the Free University of Berlin (hereafter IPFUB). Alongside the type specimen IPFUB Gui Croc 7308—which consists of a partial skull with jaws, a vertebra from the sacrum, and two osteoderms—over 400 additional specimens are known, most of them consisting of single isolated bones. Among these, the more complete specimens are IPFUB Gui Gui Croc 7352 (tail vertebrae, femur, osteoderms); 7441 (osteoderms and ulna); 7545 (dorsal vertebrae, ischia, osteoderms); 7564 (femur, humerus, osteoderms); 7634 (dorsal vertebrae, rib, osteoderms); and 8037 (cervical and dorsal vertebrae, and osteoderms). More specimens are known, but they remain unprepared. Some specimens show the marks of scavenging.
Description
As with other members of the Atoposauridae, Knoetschkesuchus is very small, with K. guimarotae measuring only long at maximum. Typical of the group, both species were quadrupedal, bearing long and slender limbs. The backs of both of the known species are covered with two rows of bony rectangular osteoderms, centred at the midline, that are wider than they are long. Each osteoderm bears a keel running longitudinally, although the keels are less-developed in K. langenbergensis. In at least K. guimarotae, the osteoderms on the tail have sharper and higher keels; the ones near the back of the tail tend to be longer than they are wide, unlike the other osteoderms, and are also vaulted. The vertebrae of K. guimarotae are additionally amphicoelous, or concave at both ends; while initially proposed as a unique trait, Theriosuchus pusillus bears amphicoelous vertebrae as well. The skull and jaws, which exhibit a number of characteristics that separate Knoetschkesuchus from other atoposaurids, are described in further detail below.
Snout
The skull of Knoetschesuchus is relatively short, with the snout taking up 47% of skull length in K. langenbergensis and 42% in K. guimarotae, which allows them to be classified as brevirostrine crocodilians. Along the side of the snout are two undulations, a smaller one on the premaxilla and a larger and broader one on the maxilla. The jagged suture between the premaxilla and maxilla is angled towards the front of the skull in K. langenbergensis and towards the back in K. guimarotae. Along the midline of the snout are the thin and wedge-like nasals; the nostrils, which face upwards, are clearly separated by the nasals in K. guimarotae, but it is not clear that this is the case in K. langenbergensis.
At the back, the nasals are separated by the frontal in K. langenbergensis, the back third of which is somewhat vaulted. The portion of the frontal between the eyes is one-third the width of the entire skull in both species, but it widens further back to form the front of the skull roof. The prefrontal is straight along its contact with the frontal and nasal (about half of the bone is in contact with each), but forms an angle between the margin of the eye socket and the lacrimal on the other side. This angle is rounded in K. guimarotae such that the bone is oval-shaped, but very pointed in K. langenbergensis such that the bone is triangular.
Eye socket and skull roof The main body of the lacrimal is a rounded square with both faces of the bone are concave. Its contact with the nasal is rather limited in both species. The oval-shaped antorbital fenestra is small, being only 9% the length of the eye socket; its presence is unique to both species of the genus among atoposaurids. Meanwhile, the eye socket is large and oval, being 54% longer than it is tall. The drop-shaped palpebrals project out from the tops of the eye sockets. In both species, the back two-thirds of the inner surface of the palpebral is slightly concave. Both species have a squamosal in which the back third is bevelled; in K. langenbergensis, the outer margin is somewhat convex.
Viewed from the top, the parietal increases in width at the back; the increase is small in K. langenbergensis, such that the bone is overall rectangular, but the difference is larger in K. guimarotae. The back of the bone bears a small notch in K. langenbergensis and a general concavity in K. guimarotae that slightly exposes the underlying supraoccipital. The postorbital bears two branches that join with a gentle curve, separated by an angle of 130° in K. langenbergensis. The supratemporal fenestra is roughly square in K. guimarotae but has a thinner back end in K. langenbergensis. In both species, the maximum distance between the supratemporal fenestrae is about a third of the total width of the top of the skull. The trapezoidal infratemporal fenestra is 1.5 times as long as it is wide in K. langenbergensis.
Palate and braincase On the bottom of the skull, the pterygoid is about twice as wide as it is long. At the front of the pterygoid is a small projection that extends backwards to form a ridge, on either side of which is a furrow-like depression (the choanal groove) containing the choanae. In Theriosuchus, unlike Knoetschkesuchus, the choanae are embedded in a wider bowl-like depression. Additionally, in both species, the bottom surface of the pterygoid is somewhat concave. Extending forward from either side of the pterygoid is the ectopterygoid; in both species, this bone is constricted near the middle to form an hourglass-like shape, but in K. langenbergensis it is also somewhat twisted to the side. The back of the bone is very concave in both species.
Forming the sides of the back of the skull is the exoccipital, which surrounds most of the foramen magnum. The Eustachian tube extends downwards across the basoccipital and basisphenoid; these bones are thickened on either side of the tube in K. langenbergensis. K. guimarotae has a small rounded foramen beside the tube on the basoccipital, and a tuberosity bearing a ridge above. On the front of the basoccipital in both species, there are two rounded depressions near the bottom. The basisphenoid resembles a triangular hatchet in shape when viewed from the side.
Jaw In K. guimarotae, the two halves of the dentary diverge from each other at an angle of 20° near the front, then 40° near the back. It also bears two convexities on the bottom of the jaw, one at the third and fourth teeth and another at the eighth to tenth teeth. The latter convexity is replaced by a concavity in K. langenbergensis. In both species, the top margin of the jaw behind the tooth row slopes upwards in a straight line. The side of the dentary is pitted, albeit much more densely so in K. langenbergensis. Near the back of the dentary in both species, the pits are replaced by longitudinal grooves. On the interior of the jaw, the splenial bears an oval foramen behind the level of the symphysis in both species, and the top of the bone bears a low and roughened crest in K. guimarotae.
The tip of the angular is situated close to the midpoint of the bone in K. langenbergensis rather than being at the back as in K. guimarotae. In both species, the back of the angular contributes to the retroarticular process. The inner surface of the angular is roughened in K. guimarotae, and the top margin of the inner wall transitions from an upward-projecting tip at the front to a low, rounded crest at the back; the same margin maintains its height along the angular in K. langenbergensis. The surangular bears a thin, forward-projecting process that, in K. langenbergensis, extends forward to the back of the tooth row and bears a groove on the bottom. The presence of the oval-shaped external mandibular fenestra is unique to the genus among atoposaurids.
Teeth