Achelata, Scholtz and Richter, 1995
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publication ID |
https://doi.org/ 10.4202/app.00627.2019 |
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persistent identifier |
https://treatment.plazi.org/id/0394879B-FFB1-686A-FCB4-5D2AFD9B6E0E |
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treatment provided by |
Felipe |
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scientific name |
Achelata |
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Achelata View in CoL gen. et sp. indet.
Figs. 1 View Fig , 2 View Fig .
Material.—SMNS 70449 (only known specimen), late larval stage with prominent exopods from Gomaringen near Tübingen, Southern Germany, Neth quarry, Unterer Stein Bed, Posidonienschiefer Formation ( Posidonia Shale ). Early Toarcian, Harpoceras falciferum Zone ( Riegraf et al. 1984). The quarry is abandoned since decades and after renaturation the section is no longer exposed.
Description.—A late larval stage with prominent exopods, but already adult-type body, proximal region of antenna and endopods of posterior thoracopods. Rather small at this stage with only about 20 mm main body length ( Fig. 3B View Fig ).
Incompletely preserved euarthropodan specimen; total length of main body axis in anterior-posterior axis plus preserved parts of anterior projections slightly more than 20 mm ( Fig. 1A View Fig ). Body elongate, about five times as long as wide (maximum width). Body subdivided into three more or less distinct regions. Anterior region without laterally projecting appendages; middle region with prominent laterally projecting appendages; posterior region without appendages, but subdivided into seven more or less distinct sections ( Fig. 1B View Fig ).
Anterior region of the body about as long as maximum width of the body. Anterior rim of the anterior region as wide as maximum body width, anterior region narrowing posteriorly. Posterior rim of the anterior region about 80% of the width of the anterior rim. Anteriorly with two projections directed forward, apparently broken off, must have been significantly longer. Proximal width of projections more than 30% of the body width. Furthermore, two more structures seen as faint impressions, most likely representing appendages, arise from close to the posterior rim of the anterior region. Thinner than the anterior projecting structures (hence smaller diameter), about 20% of the maximum body width. At least five times as long as wide. Proximally with a distinct square-shaped region, set off from the further distal one.
Middle region of the body about twice as long as anterior region. Narrower anteriorly, widening posteriorly to reach the maximum width in the middle (anterior-posterior axis), and remaining as wide towards the posterior. Middle region with five prominent appendages on each side, evenly distributed along the lateral rim. All appendages apparently sub-similar originally, yet in different states of preservation. Maximum length of an appendage about 70% of the entire body length excluding appendages; diameter about 25% of the body width.
Overall appendage morphology with a proximal main axis with two distinct elements (coxa and basipod). Coxa slightly longer along proximal-distal axis than wide. Basipod slightly longer than coxa. Basipod carrying two branches distally, the more medially placed one presumably being the endopod and the more laterally placed one presumably being the exopod. Endopod clearly subdivided into five elements. Proximal element of endopod (ischium) similar to basipod in size. Endopod element 2 (merus) significantly longer, about twice as long as preceding elements, more than twice as long as wide, slightly curved. Endopod element 3 (carpus) significantly shorter, slightly shorter than the coxa, also more slender, slightly tapering distally. Endopod element 4 (propodus) longest of the series, slightly shorter than the combined length of ischium and merus. About as slender as carpus, slightly tapering distally. Endopod element 5 (dactylus) short, slender, inward curved, about as long as carpus, but only 50% of the diameter. Exopod not well preserved, subdivision not apparent, slender. Maximum length of exopod at least as long as merus, but only about 50% of its width (diameter).
Prominent appendage 1 without exopod, either as original condition or not preserved/not visible. Prominent appendage 2 slightly longer than prominent appendage 1; also here the presence of an exopod is unclear. Prominent appendage 3 smaller than preceding, i.e,. slightly shorter and more slen- der. The propodus of prominent appendage 3 is more slender and tapering distally, the dactylus is almost as wide as the distal edge of the propodus. Exopod present. Prominent appendages 4 and 5 progressively smaller, exopods present.
Posterior region of the body with seven more or less distinct sections. First section slightly narrower than middle region, more or less rectangular in dorsal-ventral view, short, about 20% of the maximum body width. Second section wider, slightly longer and very gently curving backwards.
Section 3 about as wide as section 2, slightly longer, even more curved. Section 4 with similar dimensions as 3, but even more curved. Section 5 longer, slightly narrower, also appearing curved. Section 6 similar in dimensions to 5, but appearing more rectangular again. Section 7 sub-similar to 6. A small lobe-like structure protrudes from under section 7; width about 30% of maximum body width, half as long as wide.
Three-dimensional relief of specimen shows that the region between the prominent appendages is subdivided into six structures ( Fig. 2 View Fig ). Each appears as depression running from the insertion region of one appendage to the corresponding one on the other side. The anteriormost one appears to be corresponding to the smaller appendage visible in the anterior region.
Remarks.—The available structures preserved on the specimen provide us with enough information to allow a sound systematic interpretation. The principle body organization 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 Decapoda .
In this context, the anterior projections are best interpreted as massive antennae, and the superimposed appendages as the maxillipeds three (= thoracopods 3). 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 (e.g., Williamson 1969). 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 (Johnson 1951; Palero et al. 2014a).
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 ( Scholtz and Richter 1995). An additional character supporting this interpretation is that of the five prominent thoracopods the second one is slightly larger than the first one ( Scholtz and Richter 1995). 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 .
Overall, the specimen resembles non-phyllosoma-type fossil larvae from the Mesozoic such as Polzicaris sahelalmae from the lithographic limestone of Lebanon (Turonian?, about 90 million years old; Haug et al. 2011, 2013a) and especially “ Palinurina ” tenera Oppel, 1862 from the Solnhofen lithographic limestone (Tithonian, about 150 million years old; Haug and Haug 2016). This similarity accounts for the overall body organisation, appendage morphology with robust endopods, but retaining the exopods, and the arrangement of the thoracopod insertions, which is more oval/ circular than triangular (as in adult achelatan lobsters).
Unlike many achelatan lobsters developing via a stage with intermediate morphology between phyllosoma-type morphology and juvenile/adult-type morphology. Differing from some other comparable forms by body shape. Differing from Polzicaris sahelalmae ( Fig. 3A View Fig ), which is much larger at a comparable stage and retains the specialised phyllosoma-type claw, which is absent in SMNS 70449. Differing from “ Palinurina ” tenera mainly through size ( Fig. 3C View Fig 1 View Fig , C 2 View Fig ), the smallest known stage of “ Palinurina ” tenera is only sclerotised at its thoracic appendages, hence less far sclerotised than SMNS 70449, but much larger ( Fig. 3C View Fig 1 View Fig ). Specimens of “ Palinurina ” tenera with a comparable morphology to SMNS 70449 are even larger.
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