Alvis Octopus

''Pseudastacus'' (meaning "false ''Astacus''", in comparison to the extant crayfish genus) is an extinct genus of decapod crustaceans that lived during the Jurassic period in Europe, and possibly the Cretaceous period in Lebanon. Many species have been assigned to it, though the placement of some species remain uncertain and others have been reassigned to different genera. Fossils attributable to this genus were first described by Georg zu Münster in 1839 under the name ''Bolina pustulosa'', but the generic name was changed in 1861 after Albert Oppel noted that it was preoccupied. The genus has been placed into different families by numerous authors, historically being assigned to Nephropidae or Protastacidae. Currently, it is believed to be a member of Stenochiridae.

Not exceeding in total length, ''Pseudastacus'' was a small animal. Members of this genus have a crayfish-like build, possessing long antennae, a triangular rostrum and a frontmost pair of appendages enlarged into long and narrow pincers. Deep grooves are present on the carapace, which is around the same length as the abdomen. The carapace is usually uneven, with either small tubercles or pits across the surface. Sexual dimorphism is known in ''Pseudastacus'', with the pincers of the females being more elongated than those of the males. There is evidence of possible gregarious behavior in the form of multiple individuals preserved alongside each other, possibly killed in a mass mortality event.

With the oldest known record dating to the Sinemurian age of the early Jurassic, and possible species surviving into the Cenomanian stage of the late Cretaceous, ''Pseudastacus'' has a long temporal range and was a widespread taxon. Fossils of this animal were first found in the Solnhofen Limestone of Germany, but have also been recorded from France, England and Lebanon. All species in this genus lived in marine environments.

Discovery and naming

Fossils of ''Pseudastacus'' had been described prior to the naming of this genus, under other names which are currently invalid. In 1839, Georg zu Münster established the genus ''Bolina'' to include two species, ''B. pustulosa'' (the type species) and ''B. angusta'', both of which are based on specimens collected from the Solnhofen Limestone. The generic name references the nymph Bolina from Greek mythology. A year later, Münster described several fossils from the Solnhofen Limestone he believed to represent isopods, and erected the genus ''Alvis'' to contain the single species ''A. octopus'', naming it after the dwarf Alvíss from Norse mythology.

In 1861, Albert Oppel noted that the name ''Bolina'' was preoccupied by a genus of cnidarian, and thus the crustacean named by Münster had to be renamed. Oppel placed ''B. pustulosa'' and ''B. angusta'' into two new genera, ''Pseudastacus'' and '' Stenochirus'' respectively. Now renamed as ''Pseudastacus pustulosus'' and ''Stenochirus angustus'', the two species became the type species of their own respective genera. The name ''Pseudastacus'' means "false ''Astacus''", referencing its resemblance to the modern crayfish genus. Oppel declared that 10 specimens known at the time represented ''Pseudastacus pustulosus'', of which one was from the Redenbacher collection and the remaining nine were from the collection of the Palaeontological Museum, Munich. In addition, he identified one specimen (BSPG AS I 672) housed in the Palaeontological Museum as a second species of the genus which he named ''Pseudastacus muensteri''. In 2006, Garassino and Schweigert reviewed the decapod fossils from Solnhofen and found that four of the ''P. pustulosus'' specimens from Oppel's collection were still present, and that ''P. muensteri'' is a junior synonym of ''P. pustulosus''.

Valid species

Several species have been assigned to the genus ''Pseudastacus'', though the placement of some species remains uncertain or tentative. In addition, some have since been moved into different genera after they were discovered not to be closely related to the type species. In 2020, Charbonnier and Denis published a study including a summary of recognized stenochirid species, which covered the reclassification of former ''Pseudastacus'' species and left the following as members of the genus:

* ''P. pustulosus'' is the type species of the genus, first named as ''Bolina pustulosa'' by Münster in 1839 and moved to ''Pseudastacus'' in 1861. Its fossils were found in the Solnhofen Limestone of Germany, which date back to the Tithonian stage of the late Jurassic period.

* ''P. mucronatus'' was originally named as ''Astacus mucronatus'' by John Phillips in 1835. The type specimen was extracted from the Speeton Clay Formation, England, and is a fragment of the pincer. The chela is very large, with alternating large and small tubercles on the inner margins. This is unlike the narrower and longer pincers of other ''Pseudastacus'' species, and the specimen may be referrable to '' Hoploparia dentata''.
* ''P. minor'' was described by Oscar Fraas in 1878 from a specimen found in Cenomanian-aged deposits in Lebanon. This specimen is now lost and only the original illustration remains, which shows features unlike any other ''Pseudastacus'' species: the rostrum is extremely long, there is an additional abdomen segment, the clawed limbs are placed further back and the general pincer shape is different. Its placement in this genus is thus uncertain.
* ''P. pusillus'' is based on a fossil from the Bajocian-aged deposits of May-sur-Orne, France described in 1925 by Victor van Straelen. The fossil was destroyed in World War II and it is difficult to tell from the original line drawing of the specimen whether this species truly belongs to ''Pseudastacus''.
* ''P. lemovices'' was named in 2020 based on five specimens preserved in a slab of Sinemurian-aged limestone, collected from Chauffour-sur-Vell, France. The specific name honors the Lemovices, a Gallic tribe that lived near this locality. It is the oldest known species of the family Stenochiridae.

Reassigned species

The following species were formerly placed in ''Pseudastacus'', but have now been moved to different genera.

* ''P. hakelensis'' was first named as ''Homarus hakelensis'' in 1878. The species lived during the Cenomanian stage in Lebanon. It was moved to ''Notahomarus'' in 2017.
* ''P. dubertreti'', described in 1946 from a fossil kept in the National Museum of Natural History, France, lived in Lebanon during the Cenomanian stage. Later study of the fossil found this species to be synonymous with ''Carpopenaeus callirostris'' in 2006.
* ''P. llopisi'' was named in 1971 and is known from numerous specimens found in the early Cretaceous-aged site of Las Hoyas, Spain. In 1997 it was reassigned to the genus ''Austropotamobius''.

Description

''Pseudastacus'' is a small invertebrate, with the carapace of ''P. lemovices'' reaching a length of (excluding the rostrum) and a height of . The known specimens of ''P. pustulosus'' range from in total length.

Members of this genus often have an uneven carapace surface, with some species (such as ''P. pustulosus'') having tubercles and others (such as ''P. lemovices'') having pits distributed uniformly across the carapace surface. Individuals with smoother carapaces are also documented, though this may be due to abrasion. Grooves are present on the carapace, including a deep, arch-shaped cervical groove that stretches across the top and sides of the carapace, and an additional groove behind it on either side. The rostrum is triangular and elongated, with three lateral spines. The carapace and head are separated by an arch-shaped incision. A pair of long antennae and two pairs of shorter antennules extend from the head, with the outer antennules being slightly narrower and more pointed than the outer pair. A pair of compound eyes are attached to the head by short eye stalks.

The first three pairs of appendages terminate with chelae (pincers), and the appendages furthest front are particularly long and enlarged. The abdomen is around the length of the carapace, with the frontmost segment being the smallest. The pereiopods (walking legs) on the thorax decrease in size the further back they are placed, the pair furthest front being largest and longest. The uropods are equal in length, with a ridge down the middle and long setae on the margins.

Classification

In the centuries since it was first discovered, ''Pseudastacus'' has been placed in a wide variety of families by many different authors. For many decades, the genus was thought to be a member of Nephropidae (the lobster family), as first reported by Victor van Straelen in 1925. This placement was followed by subsequent authors such as Beurlen (1928), Glaessner (1929), and Chong & Förster (1976). In 1983, Henning Albrecht erected the family Protastacidae and moved ''Pseudastacus'' into it, whereas Tshudy & Babcock (1997) included the genus into their newly-established family Chilenophoberidae. Although Garassino & Schweigert (2006) continued to place ''Pseudastacus'' in Proastacidae following Albrecht (1983), other authors in the 2000s would place it in Chilenophoberidae based on the more recent findings of Tshudy & Babcock (1997).

In 2013, Karasawa ''et al.'' recovered ''Pseudastacus'' as the sister taxon to '' Stenochirus'', making Chilenophoberidae a paraphyletic group. The family was therefore synonymized with Stenochiridae. The following cladogram shows the placement of ''Pseudastacus'' within Stenochiridae according to the study:

Palaeobiology

Sexual dimorphism

Albert Oppel noticed that ''Pseudastacus'' fossils from the Solnhofen Limestone could be distinguished into two morphs; aside from those most similar to the ''P. pustulosus'' type specimen, there were also some with smaller bodies and longer, more slender claws. Oppel believed the latter morph to be a separate species which in 1862 he named ''P. muensteri''. Over a century later, Garassino and Guenter (2006) found that specimens of ''P. muensteri'' were essentially identical to ''P. pustulosus'' aside from the claw form. In addition, they noted that in fossil glypheids and the extant ''Neoglyphea inopinata'', the females possess longer clawed limbs than the males. Based on this, they declared ''P. muensteri'' as a junior synonym of ''P. pustulosus'', actually representing female specimens of the species.

Social behavior

The type series of ''P. lemovices'' is made up of five individuals preserved together in a single limestone slab, possibly indicating the species exhibited gregarious behaviour, with this group being killed in a mass mortality event (perhaps caused by temperature changes or lack of oxygen). Evidence of gregarious behaviour is also known in other fossil lobsters, as well as in extant species.

Palaeoenvironment

Early Jurassic

''Pseudastacus'' is believed to have first evolved during the early Jurassic, with ''P. lemovices'' being the oldest member of the genus currently known. The five known specimens of this species were preserved in a single limestone slab collected from a garden in Chauffour-sur-Vell, France. This sediment in this locality represents a marine environment dating back to the Sinemurian age, and the general area has been specifically dated to the late Sinemurian based on the presence of the green alga ''Palaeodasycladus mediterraneus'' in a regional bed.

Late Jurassic

''Pseudastacus pustulosus'', the type species of the genus, is also known from the most specimens. All known remains of this species were collected from the Solnhofen Limestone of Bavaria, Germany, which dates to the Tithonian age of the late Jurassic period. During the time of deposition, the European continent was partly inundated, forming a dry, tropical archipelago at the edge of the Tethys Ocean. The Solnhofen Limestone would have been laid down in a lagoonal environment cut off from the main ocean by reefs. A coastal habitat is further confirmed by the fossil content of the area, which includes numerous marine species that ''P. pustulosus'' would have lived alongside. These include cephalopods (such as ammonoids and belemnites), crinoids (such as '' Saccocoma''), other crustaceans (including eryonids, axiids, glypheids, mantis shrimps, and the closely related '' Stenochirus''), fish (such as pycnodonts, pachycormids, aspidorhynchids and caturids) and marine reptiles (such as turtles, ichthyosaurs and metriorhynchids). Remains of terrestrial animals, though rarer, are also present and represent species that would have lived on the islands surrounded by the lagoons, including dinosaurs (such as ''Archaeopteryx'' and ''Compsognathus''), lizards (such as ''Ardeosaurus'', ''Bavarisaurus

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