Thursday, 7 November 2019

A Spiny Lobster larvae from the Early Jurassic Posidonia Shale of southern Germany.

The Decapoda are a group of Crustaceans with a crucial impact on modern marine ecosystems, and, in the case of edible forms, are also important for the economy of certain areas. While the many forms of adult Decapods, such as Prawns, Shrimps, Lobsters, and Crabs, are well-known to most people, their larvae are less well-known to the public. Ecologically, larvae of Decapods may even be more important than their adult counterparts given their specific role in the marine food web: they are part of the plankton, preying on smaller plankton and being prey for larger organisms. Some of these larvae reach enormous sizes, remain in the plankton for a longer time span and moult into different larval stages, in some cases more than ten distinguishable ones. Probably the most impressive examples are the larval stages of Spiny lobsters, Achelata. These larvae are called phyllosoma and can reach up to 150 mm in leg span. Phyllosoma larvae are very flat, translucent and possess long and thin legs, and have very long larval phases during which they reach these extreme sizes. To remain in the water column despite their unusually large size, these larvae ride on Jellyfish and other gelatinous macro-plankton. 

While it might be expected that fragile-appearing larval forms such as a phyllosoma are impossible to be found as fossils, quite the opposite is true. Only a handful of larval forms of other groups of Decapod Crusaceans are known, while literally thousands of fossils of phyllosoma larvae have been found. Most of these immature Achelatan fossils originate from the Late Jurassic Solnhofen-type lithographic limestone beds of southern Germany and have an age of about 150 million years. The Late Cretaceous lithographic limestone of Lebanon have a comparable preservation potential and have provided us with some fossil phyllosoma-like larvae of about 90 million years (Turonian) in age, and some remains of compound eyes from the Lower Cretaceous of Brazil have also been interpreted as possible parts of phyllosoma larvae. Yet, some of the fossil Achelatan larvae from the lithographic limestone of Lebanon and southern Germany are not phyllosoma larvae in the strict sense. These fossils have combinations of characters that occur in modern phyllosoma larvae and other characters that today only occur in post-phyllosoma stages of Achelatan Lobsters. More importantly, there are different types of such larvae, each of them possessing different types of character combinations. This indicates that the diversity of larval morphologies of Achelatan Lobsters was higher in the past.

In a paper published in the journal Acta Palaeonotologica Polonica on 14 October 2019, Joachim Haug and Carolin Haug of the Biocenter and GeoBio-Center at Ludwig Maximilians University Munich, and Günter Schweigert of the State Museum of Natural History Stuttgart, describe a Phyllosoma larvae from the Early Jurassic Posidonia Shale of southern Germany, the earliest known example of such a larvae.

The Posidonia Shale outcrops across much of southern Germany, and is noted for the many exceptionally well preserved Fish and Marine Reptiles it produces, as well as Ammonites (particularly the abundant Posidonia bronni which gives the formation its name), and Crinoids. The Crustacean fauna of the Posidonia Shale is not very diverse, and fossils of Crustaceans have been considered to be rather rare. Among the known forms are the Lobster-like Uncina posidoniae, whinch reaches sizes of almost half a meter; numerous species of Polychelidan Lobsters (nowadays only known from the deep sea), among them species of Proeryon, Tonneleryon, and Coleia, rare specimens of Glypheidan Lobsters (a group nowadays only known in the fossil record from the Triassic onwards, but thought to be extinct until two species were found living in the deep Pacific), and some unclear records, among them a supposed Mantis Shrimp and a supposed specimen of a Hermit Crab, Palaeopagurus sp.

The specimen, SMNS 70449, comes from the Unterer Stein Bed at Neth Quarry at Gomaringen near
Tübingen in central Baden-Württemberg, Germany. It is a late larval stage with prominent exopods, but which already has an adult-type body, proximal region of antenna and endopods of posterior thoracopods. It is rather small at this stage being only about 20 mm in main body length. The body elongate, being about five times as long as wide, and subdivided into three more or less distinct regions, an anterior region without laterally projecting appendages, a middle region with prominent laterally projecting appendages, and a posterior region without appendages, but subdivided into seven more or less distinct sections.

 Larva of Achelata, SMNS 70449, Toarcian, Lower Jurassic, Gomaringen, Southern Germany. (A) Composite microscopic photograph under cross-polarised light. (B) Colour-marked photograph indicating the visible structures. Abbreviations: ba, basipod; cx, coxa; e1–5, endopod element 1–5; m3, maxilliped 3; p2–6, pleon segment 2–6; t4–8, thoracic appendages 4–8 ('pereiopods' 1–5). Haug et al. (2019).

The anterior region of the body is about as long as maximum width of the body. Anterior rim of the anterior region as wide as maximum body width, from here the anterior region narrows posteriorly, until the posterior rim of the anterior region is about 80% of the width of the anterior rim. At the foremost part of this region are two projections directed forward and apparently broken off, these must have been significantly longer in life. The proximal width of these projections is more than 30% of the body width. Furthermore, two more structures are seen as faint impressions, most likely representing appendages that arise from close to the posterior rim of the anterior region. These are thinner than the anterior projecting structures (hence smaller diameter), about 20% of the maximum body width. The anterior region is least five times as long as wide. Proximally it has a distinct square-shaped region, set off from the further distal one.

The middle region of the body about twice as long as anterior region, and is narrower anteriorly, widening posteriorly to reach the maximum width in the middle, and remaining as wide towards the posterior. The middle region has five prominent appendages on each side, evenly distributed along the lateral rim. All appendages were apparently sub-similar originally, but are in different states of preservation. The maximum length of an appendage is about 70% of the entire body length excluding appendages; the diameter about 25% of the body width.

The overall appendage morphology has a proximal main axis with two distinct elements (the coxa and basipod). The coxa is slightly longer along proximal-distal axis than wide. The basipod is slightly longer than the coxa. Each basipod carries two branches distally, the more medially placed one presumably being the endopod and the more laterally placed one presumably being the exopod. The endopod clearly subdivided into five elements. The proximal element of endopod (the ischium) is similar to  the basipod in size. Endopod element 2 (the merus) is significantly longer, about twice as long as preceding elements, more than twice as long as wide, and slightly curved. Endopod element 3 (the carpus) is significantly shorter, slightly shorter than the coxa, also more slender, and slightly tapering distally. Endopod element 4 (the propodus) is the longest of the series, slightly shorter than the combined length of ischium and merus, and about as slender as carpus, slightly tapering distally. Endopod element 5 (the dactylus) is short, slender, inward curved, about as long as carpus, but only 50% of the diameter. The exopod is not well preserved, subdivision is not apparent, it is slender. The maximum length of exopod at least as long as merus, but only about 50% of its width (diameter).

The posterior region of the body has seven more or less distinct sections. The first section is slightly narrower than middle region, more or less rectangular in dorsal-ventral view, and short, being about 20% of the maximum body width. The second section wider, slightly longer and very gently curving backwards. Section 3 is about as wide as section 2, slightly longer, and even more curved. Section 4 has similar dimensions as 3, but is even more curved. Section 5 is longer, slightly narrower,  and also appears curved. Section 6 is similar in dimensions to 5, but appearing more rectangular again. Section 7 is sub-similar to 6. A small lobe-like structure protrudes from under section 7. The width of the posterior region is about 30% of maximum body width. 

The available structures preserved on the specimen provide Haug et al. with enough information to allow a sound systematic interpretation. The principle body organisation, with an anterior region, i.e., head region, a middle region with five pairs of prominent appendages and a posterior trunk region with seven distinct sections, is best compatible with the interpretation of the specimen as a representative of the Decapod Crustaceans. 

In this context, the anterior projections are best interpreted as massive antennae, and the superimposed appendages as the maxillipeds (thoracopods). Five pairs of appendages of the middle region represent the posterior five thoracopods (pereiopods). The seven sections of the posterior trunk are interpreted as the six pleon segments and the telson, the lobe-like structure as a part of the uropods. 

The posterior five thoracopods appear like robust walking appendages and indicate a position within Reptantia (the group including Lobsters and Crab-like forms). This may be further supported by the fact that the specimen is preserved in dorso-ventral orientation, possibly indicating a certain original compression of the body in this direction. The depressions in the median region would correspond to elevations in the thoracic sternum still indicating the individual segmentation. There are six of these corresponding to the segments bearing maxillipeds three and the five prominent posterior thoracopods.

Among crustaceans of the group Decapoda the exopods, i.e., the outer branches of biramous appendages, become reduced during ontogeny when the organism settles to the ground. The fact that exopods are still present at least on some of the thoracic appendages at a size of about 20 mm indicates that the specimen is a larval representative of Achelata. Modern larvae of Achelata with biramous appendages can reach leg spans of 150 mm.

An identity as an achelatan lobster is further supported by the fact that thoracopod 4 does not possess a chela, but a simple curved dactylus and no finger-like extension of the propodus. An additional character supporting this interpretation is that of the five prominent thoracopods the second one is slightly larger than the first one. The prominent anterior broken-off appendages, best understood as the proximal parts of the massive antennae, further support that the specimen is a representative of Achelata. 

Based on the preserved details, Haug et al. identify the new specimen as the larva of an Achelatan Lobster. So far, fossil larvae of Achelatan Lobsters were only known from the lithographic limestone of Lebanon and Southern Germany and possibly from Brazil. Hence, the new find represents the oldest report of such a larval form. 

The specimen has a morphology that is unknown from any extant Achelatan Lobster, as it combines characters that are in modern forms characteristic for phyllosoma-type larvae (e.g., presence of exopods, circular arrangement of posterior thoracopods) and post-phyllosoma stages (e.g., robust posterior thoracopod endopods, well developed and sclerotised pleon). In its overall morphology it resembles some of the intermediate stages known from the lithographic limestone of southern Germany. Most strongly it reminds of the stages of 'Palinurina' tenera, yet these are significantly larger. The leg arrangement and the rough body outline appear also quite similar to later immature stages that have been interpreted as Cancrinos claviger. Yet, the latter has already lost ontogenetically its exopods at a comparable body size. 

 Size comparison of different non-phyllosoma type Achelatan larvae. All specimens as idealised restorations. The light grey areas represent body parts not being preserved, but inferred. (A) Polzicaris sahelalmae. (B) SMNS 70449. (C) 'Palinurina' tenera, earliest (C₁) and largest (C₂) known stage. (D) Cancrinos claviger, earlier larva still possessing exopods (D₁), later larva with exopods already absent (D₂), possible juvenile, yet without triangular sternum (D₃). Haug et al. (2019).

Specimen SMNS 70449 is a new addition to the growing group of fossil Achelatan larvae with intermediate or 'intermetamorphic' morphologies. It adds yet another subtle variation concerning the combination of characters and an additional size range, smaller than most of the known forms, but larger than the exopod-bearing stages of Cancrinos claviger. Given its age, the new larva is the oldest of these larvae, being about 30 million years older than any other known Achelatan larva. It is also the second-oldest report of a Eumalacostracan (the Eumalacostraca includes all Crustaceans except Mantis Shrimps, Ostracods, Barnacles, Copepods, Remipedes, and Branchiopods) larva so far, the oldest candidate being Mesoprosopon triasinum from the Triassic Hallstatt Limestone of Austria.

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