Form and function in Thylacocephala, Conchyliocarida and Concavicarida (?Crustacea): a problem of interpretation

Abstract

Differences in the preservation of Jurassic thylacocephalans and conchyliocarids have given rise to different interpretations of the form of these fossils, and thus their mode of life. When evidence from these two groups is combined with that derived from Palaeozoic concavicarids, it becomes possible to unify the several interpretations of this one group of organisms, the Thylacocephala. The group ranges from at least the Silurian to the Cretaceous.A review is given of how these differences of interpretation have arisen, and some resolution is attempted. If the thylacocephalan “anterior structure” is reinterpreted by analogy with hyperiid amphipods as a paired compound eye occupying most of the surface of the head, it explains its bilobed nature and the position of the stomach within the structure, but it raises the difficulty of a post-cephalic origin for the carapace. The simpler solution is preferred of regarding this structure as discrete paired eyes with a smooth cornea and subjacent crystal cones.The raptorial appendages are post-oral and post-adductor in insertion. They are therefore tentatively identified as the maxillae and maxilliped, but verification of the mandible's position is needed to test this. The postero-ventral battery of “body somites” is reinterpreted as paired protopods of abdominal limbs. A respiratory current is deduced to have entered a branchiostegal chamber ventrally, and left it posterodorsally. It is speculated that the looped linear pattern of intra-cuticular spheres in Paraostenia are photophores. The large eyes with small interommatidial angles were probably used to discern low contrast prey or carrion against a dim background. By analogy with hyperiid amphipods, it is suggested that at least some thylacocephalans were mesopelagic predators. They may have attained neutral buoyancy from their food substrate of shark and coleoids.

Thylacocephala (Arthropoda: Crustacea?) from the Cretaceous of Lebanon and implications for thylacocephalan systematics

“…Polz (1997), in his study of Mayrocaris bucculata, pointed out that tubercles found in that species were marked by small, ring-like structures with the remains of what he believed to be the bases of spines in the centre. Rolfe (1985) suggested that these features might possibly be photophores in accordance with his theory of thylacocephalans as bathypelagic creatures. Secretan (1985) provided a very plausible view of these structures as some kind of sensory organs composed of a hemispherical structure covered by carapace cuticle with a small pore as an opening to the outside.…”

“…Further definition of possible phylogenetic relationships among these taxa, such as Ostenocaris, Dollocaris and Paraostenia, will undoubtedly require more information from 'soft anatomy. ' Fourth, we see as yet little cladistic support for distinct monophyletic clades of Concavicarida and Conchyliocarida as outlined by Rolfe (1985) and Schram (1990). Although we must await more inclusive arrays of characters and taxa in future analyses of thylacocephalan phylogeny, it seems like that these will surely result in changes of higher classification within the group.…”

“…see Rolfe 1985). First, an anterior structure, which the Italians described as a muscular cephalic sac covered by sclerites (Pinna 1974;Pinna et al 1982), is referred to by almost every other author as a large compound eye.…”

The Kulm Facies of the Montagne Noire (Mississippian, southern France)

“…Beside plants (e.g. Meyer-Berthaud, 1984;Galtier et al, 1988) the nodules also contain radiolarians (see above), cephalopods (Delépine, 1935;Böhm, 1935;Korn & Feist, 2007) and arthropods (Delépine, 1935;Böhm, 1935;Rolfe, 1985). Early phosphatogenesis during diagenesis, probably around the water-sediment interface (Trappe, 1998) might explain the good preservation of the fossils within the nodules.…”

Early Carboniferous ammonoid faunas and stratigraphy of the Montagne Noire (France)

“…These have been commercially exploited in the middle of last century for phosphates, and were mined in numerous excavations on the south-eastern slope of the Puech de la Suque. Besides a rich radiolarian content (Deflandre 1946(Deflandre , 1973, the nodules are famous for their permineralised plant remains (e.g., Theron 1899; Bö hm 1935b; Genson 1940Genson , 1941Maurel 1966;Meyer-Berthaud 1984a, 1984bGaltier et al 1988;Rowe & Galtier 1989, 1990, as well as faunas with cephalopods (Blayac et al 1935a(Blayac et al , 1935bDelé pine 1935;Bö hm 1935a) and arthropods (Delé pine 1935;Bö hm 1935a;Rolfe 1985). It was Bö hm in particular who collected fossil material from this formation.…”