An anhanguerian pterodactyloid mandible from the lower Valanginian of Northern Germany, and the German record of Cretaceous pterosaurs

Abel, P., Hornung, J.J., Kear, B.P., and Sachs, S. 2021. An anhanguerian pterodactyloid mandible from the lower Valanginian of Northern Germany, and the German record of Cretaceous pterosaurs. Acta Palaeontologica Polonica 66 (Supplement to 3): S5–S12. The record of Cretaceous pterosaur remains from Germany is sparse. The material recovered to date includes the frag-mentary holotypes of Targaryendraco wiedenrothi and Ctenochasma roemeri , as well as a few isolated pterodactyloid teeth and some indeterminate skeletal elements, together with a plaster cast of a large Purbeckopus manus imprint. Here, we report the discovery of a pterodactyloid pterosaur mandible from lower Valanginian strata of the Stadthagen Formation in the Lower Saxony Basin of Northern Germany. Based on the size and spacing of its alveoli, this fossil is attributable to the cosmopolitan Early Cretaceous pteranodontoid clade Anhangueria. Moreover, it represents the first and only known pterosaur from the Valanginian of Germany and is one of only a handful Valanganian pterosaur occurrences presently recognized worldwide. In addition to the approximately coeval Coloborhynchus clavirostris from the Hastings Bed Group of southern England, the Stadthagen Formation pterosaur mandible is among the stratigraphically oldest identifiable anhanguerians.


Introduction
Pterosaurs were the first vertebrates to evolve an active volant lifestyle and were globally widespread from the Middle Triassic until their extinction at the end of the Cretaceous (e.g., Butler et al. 2009;Longrich et al. 2018).Among the paraphyletic succession of early pterosaur lineages, only the "short-tailed" pterodactyloids and anurognathids survived beyond the Jurassic-Cretaceous boundary, with pterodactyloids subsequently diverging into the Azhdarchoidea and Pteranodontoidea (Kellner 2003;Hone 2020).One of the most successful of these Cretaceous pterodactyloid clades were the Anhangueria, a cosmopolitan radiation of toothed and often large pteranodontoids, best known from the Aptian-Albian strata of Great Britain and Brazil (Rodrigues and Kellner 2013), but ranging up into the Turonian elsewhere (Rodrigues and Kellner 2013;Pentland et al. 2019, Frey et al. 2020).To date, the stratigraphically oldest known anhanguerian is Coloborhynchus clavirostris Owen, 1874 from the upper Berriasian-lower Valanginian of England (Rodrigues and Kellner 2013;Pentland et al. 2019), which suggests possible origination of the group shortly after the Jurassic-Cretaceous transition.
Here we describe a new fragmentary mandible (RE 551.763.120A 0333/1) of an anhanguerian pterodactyloid from the lower Valanginian of Northern Germany.This specimen was found by fossil collector and preparator Karl-Heinz Hilpert (formerly at the Geologisch-Paläontologisches Museum Münster) in the abandoned Sachsenhagen clay pit in Lower Saxony, and subsequently donated to the Ruhr Museum Essen in 1998.RE 551.763.120A 0333/1 is important, because together with C. clavirostris it represents one of the oldest known anhanguerian occurrences.In addition, it is one of only very few pterosaur specimens currently identified from the German Cretaceous (von Meyer 1851;Wild 1990;Hornung and Reich 2013;Lanser 2015) and is also the first documented pterosaur fossil found in Germany that is of Valanginian age.

Geological setting
The Sachsenhagen clay pit is located approximately 30 km west of Hannover (Fig. 1A) and was operated as brickworks from 1904 to 1986.Most of the quarry area has since been backfilled and used as a waste dump.The exposed profile spans a lithofacially mostly homogenous, pelitic succession of the upper Berriasian upper Isterberg Formation and lower Valan ginian lower Stadthagen Formation (Fig. 1B).The transition between these units correlates with a shift from euryhaline to stenohaline conditions following a marine transgression (Kaufmann et al. 1980;Mutterlose 2017).Paleo geo graphically, the deposits were formed in a shallow gulf that inundated the Lower Saxony Basin as a southward exten sion of the North Sea Basin (Fig. 1C; see Mutterlose 2017).
Description.-Thepreserved mandible includes the posterior section of the symphysis and parts of both dentaries and angulars.The splenials have been disarticulated and are now lost.Thus, the Meckelian groove is exposed on the medial surfaces of both rami (Fig. 2).The mandible has a sub-circular cross-section anterior to the symphyseal contact.There is no evidence of a sagittal crest.The external ventral face of the mandibular rami is shallowly convex.The dorsal side of the symphyseal portion is strongly concave and has an irregular midline suture (Fig. 2A).A conspicuous lateromedial furrow is visible along the length of the rami ventral to the alveolar row.The post-symphyseal space is filled with sediment.However, both rami diverge at an angle of about 23° relative to the longitudinal axis.
The mandible has suffered some diagenetic crushing, resulting in a prominent longitudinal crack on the lateral surface of the left ramus (Fig. 2B).Especially the lateral surface of the right ramus forms two longitudinal bulges, resulting in an undulating surface.The postsymphyseal portions of both rami were previously broken und glued back to the symphysis.Finally, the entire surface area of bone and exposed matrix on RE 551.763.120A 0333/1 has been varnished and painted at some stage of preparation.This has created a pattern of irregular grooves and ridges along the ventral face of the symphysis.We consider these to be artificial, yet some bulges and rugosities on the median ventral face of the symphysis and posterior parts of the rami could possibly be pathological (Fig. 2C).
Dentary: The dentaries form the mandibular sulcus, alveolar row, and the base of the Meckelian groove.The dorsal trace of the splenial suture is located ventrally adjacent to the alveoli.The ventral suture is not visible.The angular contacts are exposed on the lateral surfaces of the mandibular rami (Fig. 2B, D), however, there is no evidence of the coronoid contacts.On the right ramus, thin ridges extending parallel to the alveolar row and posterodorsally across the lateral surface posterior to the fifth alveolus (Fig. 2D) might border nerve and/or blood vessel channels.
Angular: The angulars form about the ventrolateral third of the external surfaces of the rami, and taper to the level of the posterior-most symphyseal alveoli.Medially, the angu- Alveoli: Five alveoli are preserved on the left ramus, and six on the right; only two alveoli are placed adjacent to the symphysis on either side.They are about 1.5-1.9times longer than wide, and each alveolus is slightly anteromedially directed (Table 1).The alveolar margins are flush with the surrounding bone.The interalveolar spaces are 2.5-4 times the length of the alveoli and increase in length posteriorly to the second preserved alveolus, but then decreases again further posteriorly (Table 1).
By contrast, the alveolar morphology of RE 551.763.120A 0333/1 is very similar to known anhanguerians, which typically have well-spaced alveoli along the mid-and pos- terior sections of the mandible (e.g., Wellnhofer 1987;Elgin and Frey 2011;Rodrigues and Kellner 2013).A taxonomic assignment of RE 551.763.120A 0333/1 within Anhangueria, however, proves to be difficult.Especially the taxa of the specimen-rich Cambridge Greensand and other Lower Cretaceous strata of Great Britain are often only based on anterior rostral fragments, prohibiting a direct comparison with RE 551.763.120A 0333/1 (Rodrigues and Kellner 2013), where these elements are missing.This includes most notably the roughly coeval Coloborhynchus clavirostris from the adjacent British Wealden.But even in more completely known mandibles most diagnostic traits are restricted to the anterior part of the symphysis (e.g., Steel et al. 2005).
Unlike RE 551.763.120A 0333/1 and "Santanadactylus araripensis" (Wellnhofer 1985), the angulars in Ceara dactylus atrox and Anhanguera piscator do not extend as far as the posterior end of the tooth row (Leonardi and Borgomanero 1985;Kellner and Tomida 2000).The condition of RE 551.763.120A 0333/1 is closer to "Santanadactylus araripensis", where the lateral portion of angular reaches almost the level of the posterior-most alveolus, which is in this taxon the fourth post-symphyseal tooth (Wellnhofer 1985).The same author interpreted the angular in "Araripesaurus santanae" to extend distinctly beyond the tooth bearing portion of the mandible, reaching at the least the level of the in anterior direction fifth alveolus.However, Kellner and Tomida (2000) argued that the angular may be significantly shorter in both "Santanadactylus araripensis" and "Araripesaurus santanae" but based this on the assumption of a congeneric relationship with Anhanguera piscator.This cannot be confirmed, because "Santanadactylus araripensis" and "Araripesaurus santanae" are considered to be nomina dubia (Pinheiro and Rodrigues 2017).Irrespectively, the angular of RE 551.763.120A 0333/1 is most closely comparable to "Santanadactylus araripensis" as described by Wellnhofer (1985), although the anterior extremity is less anteriorly tapered in RE 551.763.120A 0333/1.
Overall, complete mandibles are only barely known in anhanguerians, and often the mandibular bones are fused beyond distinction, suggesting these specimens may represent adult individuals (Kellner and Tomida 2000;Vila Nova et al. 2014).Evidently, RE 551.763.120A 0333/1 likely derived from an osteologically immature animal.
Cretaceous pterosaurs from Germany.-Germany is world famous for its record of articulated Upper Jurassic pterosaur remains (e.g., Wellnhofer 1970Wellnhofer , 1978;;Bennett 2002Bennett , 2007;;Tischlinger 2010;Hone et al. 2013;Tischlinger and Frey 2013).Conversely, pterosaur fossils of Cretaceous age are extremely rare despite the earliest documented occurrences dating back to the 19 th century.This is also in stark contrast to other Cretaceous pterosaur-bearing sites in the world with a rich pterosaur diversity like the northeastern Brazilian Santana Group (e.g., Wellnhofer 1985Wellnhofer , 1987;;Kellner and Tomida 2000;Unwin 2002), or the British Wealden and Cam bridge Greensand (e.g., Unwin 2001;Rodrigues and Kellner 2013;Rigal et al. 2017).The German record is currently restricted to the Lower Cretaceous.However, only the Valanginian and Hauterivian pterosaur specimens derive from strata that were deposited in some distance from the paleo-coastline.All other remains originated from non-marine, lagoonal to fully limnic and fluviatile environments.
The first Cretaceous pterosaur find described from Germany has been the holotype of Ctenochasma roemeri (non-catalogued, von Meyer 1851) from the lower Berriasian Münder Formation of the Deister region of Lower Saxony.It consisted only of a fragmentary mandible, but unfortunately the original specimen in the collection of Clausthal is not traceable (SS personal observation).
Similarly, Hornung and Reich (2013) described a right manus track (GZG.IF.00102), assignable to the large pterosaur ichnotaxon Purbeckopus cf.pentadactylus Delair, 1963.This positive plaster cast derived from an imprint found in upper Berriasian silicilastics of the Deister Formation near Bückeburg and was made sometime around 1935.However, the original may have never been collected, and the cast was likely produced in situ.
A fragmentary wing phalanx and jaw elements (GZG.STR.50291, GZG.STR.50292,RPMH "Wd.025") from the upper Berriasian Fuhse Formation near Sehnde in Lower Saxony have yet to be described in detail (Hornung 2013).Additional isolated remains have been recovered from upper Valanginian-lower Hauterivian deposits in the Leine Uplands of Lower Saxony and include what was identified as a metacarpal IV and holotype of "Ornithocheirus hilsensis" (Koken, 1883).However, a pterosaurian affinity was disputed (e.g., von Meyer 1884;Williston 1885Williston , 1886; but see also Koken 1885Koken , 1886)), and the bone has recently been recognized as the pedal phalanx of a theropod dinosaur (Hornung 2020).Today "O.hilsensis" is considered a nomen dubium and the whereabouts of the specimen are unknown (Hornung 2020).
The comparatively best-preserved pterosaur from the Cretaceous of Germany is the holotype of Targaryendraco wiedenrothi (SMNS 56628) from the lowermost Hauterivian of the Stadthagen Formation near Hannover in Lower Saxony.Wild (1990) initially named it Ornithocheirus wiedenrothi, although its assignment to the "wastebasket" genus Ornithocheirus Seeley, 1869, is widely debated (Fletcher and Salisbury 2010;Ford 2013;Rodrigues and Kellner 2013;Abel et al. 2019;Pêgas et al. 2019).Fletcher and Salisbury (2010) grouped it in Ornithocheiridae close to Aussiedraco molnari Kellner, Rodrigues, andCosta, 2011, andanother indeterminate specimen. Ford (2013) considered the species to be a "lonchodectid" and recombined it as Lonchodectes wiedenrothi.However, Rodrigues and Kellner (2013) argued against the validity of Lonchodectes and an affinity of the specimen as Ornithocheirus to be indeterminable, whereas Abel et al. (2019) reiterated a "lonchodectid" affinity.In a subsequent publication, Pêgas et al. (2019) redescribed the material as Targaryendraco wiedenrothi and found it to nest outside of Anhangueria, close to Aussiedraco molnari and Barbosania gracilirostris Elgin andFrey, 2011. Lastly, Lanser (2015) reported several isolated teeth of putative ornithocheirids from various middle Barremianlower Aptian karst-fillings in the Sauerland, North Rhine-Westphalia.
RE 551.763.120A 0333/1 adds to this sparse record as the currently only recognized Valanginian pterosaur.Furthermore, it is one of only a handful Valanginian pterosaur remains in the world (Barrett et al. 2008;Cadena et al. 2020).Of these, only the probable Valanginian C. clavirostris can be confidently assigned to Anhangueria, although, other fragmentary finds may represent anhanguerians as well (Cadena et al. 2020).Finally, RE 551.763.120A 0333/1 is, next to the teeth described by Lanser (2015), the only clear evidence for anhanguerian pterosaurs found in Germany.
Pterosaurs most frequently occur in limnic, lagoonal, and pelagic paleoenvironments (e.g., Wellnhofer 1970;Wang and Zhonghe 2006;Hone et al. 2018).Given that such depositional settings are typical of the German Cretaceous, the observed scarcity of pterosaur fossils is likely not reflecting their paleoecology, but rather the preservational biases of their fragile bones, and the reduction of Valanginian carbonate platforms in conjunction with a global cooling (Gréselle and Pittet 2010), which might have hindered the formation of Konservatlagerstätten (Butler et al. 2009).This may also explain the globally sparse record of Valanginian pterosaurs.Indeed, sampling is another evident limiting factor relative to other European Early Cretaceous pterosaur assemblages (e.g., the Cambridge Greensand, Unwin 2001;Ford and O'Connor 2004), and suggests that future exploration will likely yield new discoveries.The British Coloborhynchus clavirostris is the only other confidently assigned anhanguerian taxon from the res pe ctive time interval, representing together with RE 551.763.120A 0333/1 the oldest known records of this group.

Conclusions
The sparseness of the German Cretaceous pterosaur record may be best explainable by sampling and preservation biases and does not reflect a paleobiological signal.
RE 551.763.120A 0333/1 is important because it represents the only unambigious pterosaur fossil of Valanginian age recovered from Germany to date.RE 551.763.120A 0333/1 is among only a handful of Valan ginian pterosaur occurrences yet identified worldwide.

Table 1 .
Linear measurements (in mm) of RE 551.763.120A 0333/1.Alveoli measured in posterior direction.Asterisks indicate incomplete alveoli and/or alveoli filled with sediment.
lars border the Meckelian groove and presumably sutured to the splenials dorsally.