Tuesday, January 10, 2017

[Paleontology • 2016] Reappraisal of Europe’s Most Complete Early Cretaceous Plesiosaurian: Brancasaurus brancai Wegner, 1914 from the “Wealden Facies” of Germany


Brancasaurus brancai in its natural habitat with pycnodontiform fish; Caturus and Hybodus in the far background

Artwork by Joschua Knüppe.    Hyrotrioskjan.Deviantart.com

Abstract

The holotype of Brancasaurus brancai is one of the most historically famous and anatomically complete Early Cretaceous plesiosaurian fossils. It derived from the Gerdemann & Co. brickworks clay pit near Gronau (Westfalen) in North Rhine-Westphalia, northwestern Germany. Stratigraphically this locality formed part of the classic European “Wealden facies,” but is now more formally attributed to the upper-most strata of the Bückeberg Group (upper Berriasian). Since its initial description in 1914, the type skeleton of B. brancai has suffered damage both during, and after WWII. Sadly, these mishaps have resulted in the loss of substantial information, in particular many structures of the cranium and limb girdles, which are today only evidenced from published text and/or illustrations. This non-confirmable data has, however, proven crucial for determining the relationships of B. brancai within Plesiosauria: either as an early long-necked elasmosaurid, or a member of the controversial Early Cretaceous leptocleidid radiation. To evaluate these competing hypotheses and compile an updated osteological compendium, we undertook a comprehensive examination of the holotype as it is now preserved, and also assessed other Bückeberg Group plesiosaurian fossils to establish a morphological hypodigm. Phylogenetic simulations using the most species-rich datasets of Early Cretaceous plesiosaurians incorporating revised scores for B. brancai, together with a second recently named Bückeberg Group plesiosaurian Gronausaurus wegneri (Hampe, 2013), demonstrated that referral of these taxa to Leptocleididae was not unanimous, and that the topological stability of this clade is tenuous. In addition, the trait combinations manifested by B. brancai and G. wegneri were virtually identical. We therefore conclude that these monotypic individuals are ontogenetic morphs and G. wegneri is a junior synonym of B. brancai. Finally, anomalies detected in the diagnostic features for other “Wealden” plesiosaurians have prompted reconsiderations of interspecies homology versus intraspecific variability. We therefore propose that the still unresolved taxonomy of B. brancai should emphasize only those character states evident in the examinable fossil material, and specifically accommodate for growth-related modifications delimited via osteologically mature referred specimens.


Figure 3: Brancasaurus brancai, GPMM A3.B4 (holotype).
(A) Cranium and mandible in lateral view, showing its condition in the late 1980s. (B) Reconstructed cranium and mandible in lateral and dorsal views; recovered components identified by Wegner (1914) (blue); (C) components restored in the present mount (orange). 
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Conclusions

The holotype specimen of Brancasaurus brancai from the uppermost strata of the Bückeberg Group (upper Berriasian) of northwestern Germany is one of the anatomically most complete Early Cretaceous plesiosaurian fossils known from Europe. Since its initial description in 1914, the specimen has suffered severe damage. Nonetheless a unique combination of diagnostic traits is present, including: rectangular conjoined frontals with a concave dorsal surface and ventrally confluent lateral sides; parietals forming a parietal table; cranial and middle cervicals with distinctly triangular neural spines; dorsal transverse processes bearing subdiapophyseal fossae; scapula with a prominent lateral shelf; pelvic bar formed by the pubes and ischia; and craniolateral cornua present at the pubes. Pointedly, the holotype specimen of B. brancai was ostologically immature, as indicated by the unfused neural arches and vertebral centra. However, other features (e.g., presence of cornuae on the pubes, and well defined epipodial facets on the propodials) indicate expression of at least ‘sub-adult’ character state development. Another but more incomplete plesiosaurian skeleton from the B. brancai type locality in the upper Bückeberg Group has been named Gronausaurus wegneri, but likely represents a more mature conspecific individual. Some variation is present in the number of dorsal/sacral vertebrae. Our phylogenies otherwise detected character state conflict only in the height of the cervical neural spines, proportions of the cervical centra, and basal constriction of the dorsal neural spines. Nevertheless, these constituted polymorphisms that probably reflect specimen completeness and/or differing ontogenetic stage, suggesting that G. wegneri represents a junior synonym of B. brancai. Finally, in our opinion, the weakly supported alternative topological nesting of B. brancai + G. wegneri either within Leptocleididae, or interpolated between Elasmosauridae and Leptocleididae + Polycotylidae dictates that the taxonomic affinities of B. brancai must, at present, remain provisional.


Sven Sachs​, Jahn J. Hornung and Benjamin P. Kear. 2016. Reappraisal of Europe’s Most Complete Early Cretaceous Plesiosaurian: Brancasaurus brancai Wegner, 1914 from the “Wealden Facies” of Germany. PeerJ. 4:e2813. DOI: 10.7717/peerj.2813

"Brancasaurus - a forgotten and beaten beauty" by Hyrotrioskjan hyrotrioskjan.deviantART.com/art/Brancasaurus-a-forgotten-and-beaten-beauty-564992991

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