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The Thylacocephala (from the Greek language θύλακος or thylakos, meaning "Bag", and κεφαλή or cephalon meaning "head") are group of extinct probable Mandibulata , that have been considered by some researchers as having possible crustacean affinities. As a class they have a short research history, having been erected in the early 1980s.
They typically possess a large, laterally flattened carapace that encompasses the entire body. The tend to be large and bulbous, and occupy a frontal notch on the carapace. They possess three pairs of large raptorial limbs, and the abdomen bears a battery of small swimming limbs. Their size ranges from ~15 mm to potentially up to 250 mm.
Inconclusive claims of thylacocephalans have been reported from the lower lower Cambrian ( Zhenghecaris), but later study considered that genus as Radiodonta or arthropod with uncertain systematic position. The oldest unequivocal fossils are Upper Ordovician and Lower Silurian in age. As a group, the Thylacocephala survived to the Santonian stage of the Upper Cretaceous, around 84 million years ago.
Beyond this, there remains much uncertainty concerning fundamental aspects of the thylacocephalan anatomy, mode of life, and relationship to the Crustacea, with whom they have always been cautiously aligned.
The accuracy of this scheme has been questioned in recent papers, as it stresses differences in the arthropod eye and Exoskeleton structure, which – in modern arthropods – tend to be a response to environmental conditions. Thus it has been suggested these features are too strongly controlled by external factors to be used alone to distinguish higher taxa. The problem is exacerbated by the limited number of thylacocephalan species known. More reliable anatomical indicators would include segmentation and appendage attachments (requiring the internal anatomy, currently elusive as a result of the carapace).
Beyond this there is a lack of knowledge about even basic thylacocephalan anatomy, including the number of posterior segments, origin of the raptorials, number of cephalic appendages, shape and attachment of gills, character of mouth, stomach and gut. This results from the class's all–encompassing carapace, which prevents the study of their internal anatomy in fossils.
Of the features which could prove crustacean affinities, the arrangement of mouthparts would be the easiest to find in the Thylacocephala. The literature features some mention of such a head arrangement, but none definitive. Schram reports the discovery of mandibles in the Mazon Creek thylacocephalan Concavicaris georgeorum. Secrétan also mentions – with caution – possible mandibles in serial sections of Dollocaris ingens, and traces of small limbs in the cephalic region (not well preserved enough to assess their identity). Lange et al. report a new genus and species, Thylacocephalus cymolopos, from the Upper Cretaceous of Lebanon, which has two possible pairs of antennae, but note the possession of two pairs of antennae alone does not prove the class occupies a position in the Crown group Crustacea.
Despite a lack of evidence for a crustacean body plan, several authors have aligned the class with different groups of crustaceans. Schram provides an overview of possible affinities:
A study in 2022 describing a new arthropod from Wisconsin, Acheronauta, found that the Thylacocephalans occupied a position more primitive than the crustaceans and Myriapoda as basal stem-group Mandibulata. This would place them outside of the crustaceans as a more basal branch of the arthropod family tree.
This cladogram represents the placement of the Thylacocephalans within the arthropoda as suggested by Pulsipher, 2022.
Instead the authors suggest the sac is used to break down coarse chunks of food and reject indigestible portions.
All other parties interpret this as a large compound eye, the hexagons being preserved ommatidia (all researchers agree these are the same structure). This is supported by fossils of Dollocaris ingens which are so well preserved that individual retinula cells can be discerned. The preservation is so exceptional that studies have shown the species' numerous small ommatidia, distributed over the large eyes, could reduce the angle between ommatidia, thus improve their ability to detect small objects. Of the arguments above, it is posited by opponents that eyes are complex structures, and those in the Thylacocephala display clear and numerous affinities with compound eyes in other arthropod fossils, down to a cellular level of detail. The 'cephalic sac' structure itself is poorly preserved in Osteno specimens, a possible reason for interstitial 'sclerites'. The structural analogy with a cirripede peduncle lost supporting evidence when the 'ovaries' were shown to be alimentary residues, and the sac muscular system could be used to support the eyes. The unusual position of the stomach is thus the strongest inconsistency, but the Thylacocephala are defined by their unusual features, so this is not inconceivable. Further, Rolfe suggests the eyes' position can be explained if they have a large posterior area of attachment, while Schram suggests that the stomach region extending into the cephalic sac could result from an inflated foregut or anteriorly directed caecum.
Discussion of the matter has ceased in the last decade, and most researchers accept the anterior structure is an eye. Confusion is most likely the result of differing preservation in Osteno.
Secrétan suggested Dollocaris ingens was too large to swim, so inferred a predatory 'lurking' mode of life, lying in wait on the sea bed and then springing out to capture prey. The author also suggested it could be necrophagous, supported by Alessandrello et al., who suggest they would have been incapable of directly killing the shark remains found in the Osteno specimens' alimentary residues. Instead they surmise the Thylacocephala could have ingested shark vomit which included such remains.
Vannier et al. note the Thylacocephala possess features which would suggest adaptations for swimming in dim-light environments – a thin, non-mineralized carapace, well-developed rostral spines for possible buoyancy control in some species, a battery of pleopods for swimming, and large prominent eyes. This is supported by the Cretaceous species from Lebanon, which show adaptations for swimming, and possibly schooling.
Rolfe provides many possibilities, but concludes a realistic mode of life is mesopelagic, by analogy with Hyperiidea Amphipoda. Further suggests floor-dwelling is also possible, and that the organism could rise to catch prey during the day and return to the sea floor at night. Another notable proposal is that, like hyperiids, the class could gain oil from their food source for buoyancy, an idea supported by their diet (known from stomach residues containing shark and coleoid remains, and other Thylacocephala).
Alessandrello et al. suggest a head-down, semi-sessile life on a soft bottom, in agreement with that of Pinna et al., based on cirripede affinities. A necrophagy diet is suggested.
Briggs & Rolfe report that all the Gogo Formation Thylacocephala are found in a reef formation, suggesting a shallow water environment. The authors speculate that due to the terracing of the carapace an infaunal mode of life is possible, or the ridges could provide more friction for hiding in crevices of rock.
Schram suggests a dichotomy in size of the class results from different environments; larger Thylacocephala could have lived in a fluid characterized by turbulent flow, and relied on single power stroke of trunk limbs to position themselves. He suggests that smaller forms may have resided in a viscous medium, characterized by laminar flow, and used a lever to generate the speed necessary to capture prey.
Fossil evidence from the Holy Cross Mountains of Poland shows that thylacocephalans were regularly preyed on by predatory jawed fish during the Late Devonian epoch.
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