Poekilopleuron

''Poekilopleuron'' (meaning "varied ribs") is a genus of carnosaurian theropod dinosaur, which lived during the middle Bathonian of the Jurassic, about 168 to 166 million years ago. The genus has been used under many different spelling variants, although only one, ''Poekilopleuron'', is valid. The type species is ''P. bucklandii'', named after William Buckland, and many junior synonyms of it have also been erected. Little material is currently known, as the holotype was destroyed in World War II, although many casts of the material still exist.

Discovery and naming

''Poekilopleuron'' is a genus of theropod with a long and complex history. The holotype was discovered in July 1835 near La Maladrerie and the genus was named and first described by Jacques Amand Eudes-Deslongchamps in a report published in 1836, based on holotype material that is now destroyed. In 1837, Eudes-Deslongchamps published a more detailed account of this discovery in a monographEudes-Deslongchamps J.-A. (1837) Mémoire sur le Poekilopleuron bucklandii, grand saurien fossile, intermédiaire entre les crocodiles et les lézards. A. Hardel, Caen, 114 p., 8 pl. which was also inserted next year in volume 6 of the "Mémoires de la Société Linnéenne Normandie".

The holotype, which was housed in Musée de la Faculté des Sciences de Caen and destroyed during World War II, included gastralia, phalanges, a left forelimb, caudal vertebrae, chevrons, ribs and a hindlimb. Of all the material, little is still preserved, although the gastralia, phalanges and forelimb were cast and now represent the plastotype, with casts in the Muséum National d'Histoire Naturelle (specimen MNHN 1897-2) and Yale Peabody Museum (specimen YPM 4938). The original material was uncovered in a layer of the Calcaire de Caen in Normandy, France. ''Poekilopleuron'' can be assigned to the middle Bathonian in age, about 167.7 to 166 million years ago.

In the same 1836 publication, Eudes-Deslongchamps also named the type species of ''Poekilopleuron'', ''P. bucklandii''. Eudes-Deslongchamps noted similarities with some material of ''Megalosaurus bucklandii'' and ''Poekilopleuron'', and chose the species name ''bucklandii'' for ''Poekilopleuron'', so that if the two genera were synonymized, only the genus name would be suppressed. The generic name is derived from Greek ποίκιλος, ''poikilos'', "varied", and πλευρών, ''pleuron'', "rib", a reference to the three types of rib present. The specific name, honouring William Buckland, was deliberately identical to that of ''Megalosaurus bucklandii''.

Description

The most distinctive feature of ''Poekilopleuron'' were its forelimbs. Their length, about 60 cm, was a sign of this theropod's more original build. Unlike later Theropoda, whose forelimbs tended toward reduction in length in proportion to the animals' size, ''Poekilopleuron''s were long and, by implication, potent. The length mostly resided in the elongated but powerfully muscled humerus. The antebrachia (forearms) were markedly short and robust, a characteristic shared with ''Poekilopleuron's'' slightly later and considerably larger American cousin ''Torvosaurus''. A unique feature is a lack of the olecranon process on the ulna.

The fossil of ''Poekilopleuron'' showed a rare complete set of gastralia: fourteen pair of belly ribs supported the body of the animal.

Cau (2024) suggested that the contemporary ''Dubreuillosaurus'' (originally assigned to ''Poekilopleuron'') may represent a junior synonym, attributing the skeletal differences as positional and individual variation, as seen in ''Allosaurus''. Thus, ''Dubreuillosaurus'' would represent a less mature individual of ''Poekilopleuron''.

Classification

''Poekilopleuron'' is a difficult taxon to classify, as its original material is lost, and few casts are known. ''Poekilopleuron'' has a history of being renamed under different species and genera, most of which are now its junior synonyms.

Eudes-Deslongchamps thought the specimen might well be proven to belong to this earlier named species; if so, merely the generic name would have to be changed. Indeed, following 1879 ''Poekilopleuron'' was often subsumed under ''Megalosaurus bucklandii''. Eudes-Deslongchamps' choice caused problems however, when Friedrich von Huene in 1923 concluded it was part of ''Megalosaurus'' but as a separate species within that genus. As both species carried the same epithet ''bucklandii'', they could no longer be distinguished. Von Huene therefore renamed the species into ''Megalosaurus poekilopleuron''. Most later authors continued using the generic name ''Poekilopleuron''.

Another problem was caused by the fact that the name was only partially Latinised. In correct Greek it would have been "poikilopleuron", in Latin "poecilopleurum". This induced later writers to improve the spelling, leading to such variants as ''Poecilopleuron'' and ''Poikilopleuron'' (still used as late as 2006). However, the original name has priority and is valid.

Five other species would be named in the genus. In 1869 Edward Drinker Cope renamed '' Laelaps gallicus'' into ''Poekilopleuron gallicum''. In 1870 Joseph Leidy created a ''Poicilopleuron valens'' based on a fossil probably belonging to ''Allosaurus''. In 1876 Richard Owen named a ''Poikilopleuron pusillus'', in 1879 renamed by Cope to ''Poekilopleuron minor''; in 1887 Harry Govier Seeley made it a separate genus: '' Aristosuchus''. In 1883 W.A. Kiprijanow created a ''Poekilopleuron schmidti'', of which the specific name honours Friedrich Schmidt, based on some indeterminate ribs and a sauropod metatarsal. This chimaera is a ''nomen dubium''. A much later named species is ''Poekilopleuron valesdunensis'' created by Ronan Allain