Thylacocephala
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Contributions to Zoology, 71 (4) 159-169 (2002) SPB Academic Publishing bv, The Hague Possible lattice in Cretaceous organs Thylacocephala Sven Lange & Frederick+R. Schram Institutefor Biodiversity and Ecosystem Dynamics, University ofAmsterdam, Mauritskade 57, 1092 AD Amsterdam, Netherlands Key words: Thylacocephala, fossils, lattice organs, Thecostraca, Cirripedia, Ascothoracida, Crustacea, Cretaceous. Abstract form has come to be recognized (Pinna et al. 1982; Secretan 1983; Briggs & Rolfe 1983). Typically a reminiscent of the lattice in thecostracan the entire This Structures, organs large carapace envelops body. cara- described from the cuticle ofCretaceous crustaceans, are carapace pace covers the major part of the head and trunk, The lattice like thylacocephalans. new organ structures occur thus obscuring potentially important information similar in pairs along the dorsal midline. While these have a such as Out from under the outline lattice lack do segmentation. carapace to true organs, they seem to pores and two sets of limbs Secretan 1985; Rolfe not occur in the highly apomorphicpattern found in thecostracans. protrude (see three These discrepancies do not easily support earlier ideas of a 1985): a set of pairs of sub-chelate raptorial of the within the thecostracans. position thylacocephalans limbs, and another set forming a posterior battery the lattice for The significance of possible organs inferring a of usually eight (but sometimesmore) small swim- relationship between thylacocephalans and thecostracans is ming limbs. Recently, Lange et al. (2001) described discussed. another set of limbs corresponding to antennules and antennae. In addition to these limbs, a pair of often out from the compound eyes bulge an- Contents large terior of the A of feather- margin carapace. row like near the middle Abstract 159 gills regularly appears region Introduction 159 of the body under the carapace (Rolfe 1985; Secretan review 160 Lattice organs: a & Riou 1983; Schram et al. 1999). Thylacocephala Materials and methods 160 are known from the Silurian (Mikulic et al. 1985; Results 161 Van den Brugghen et al. 1997) through to the Cre- Presence and of lattice 161 position possible organs taceous et al. of in (Schram 1999). Morphology possible lattice organs light microscopy 163 The taxonomic affinities of the thylacocephalans in SEM 163 Morphologyof possible lattice organs remained ‘uncertain’ (Rolfe 1992), although most Reconstruction ofthe morphology ofpossible lattice authors at least implied some kind of crustacean 165 organs affinity. With the recent description of two pairs Discussion 166 of antennae in the Cretaceous thylacocephalan Thy- Acknowledgements 168 al. the References 168 lacocephalus cymolopos Lange et 2001, pos- sible affinity with Crustacea has beenstrengthened. determinationof sister However, a crustacean group Introduction for the Thylacocephala remains vexing. A number of possible and contradictory crustacean relatives Arthropod fossils now placed within the higher have, nevertheless, been suggested based on vari- taxon Thylacocephala have been known since the ous morphological similarities. Secretan (1983) 19th It is only recently that their unique century. considered the possibility of phyllocarids, mala- 160 S. Lange & F.R. Schram — Possible lattice organs in Cretaceous Thylacocephala and conchostracans sister Schram lattice while costracans as taxa. organs a varying number of ‘extra’ drew with malacostracans based fields have been in the of (1990) comparisons pore registered carapace the sub-chelate on presence of large raptorial limbs other species. Such ‘extra’ fields resemble lattice and but (stomatopods), large eyes well-developed gills organs display a more irregular layout and lack and noted the terminal found in ‘real’ lattice (many eumalacostracans), even some pores most or- potential points of similarity between thylaco- gans (Jensen et al. 1994a). and Arduini et al. first The outline ofthe lattice varies from cephalans remipedes. (1980) organs very drew attention to similarities with thecostracan cir- elongate to roundish; and from a deep depression ripedes, and this was reinforced by Pinna et al. that contains a central raised smooth keel, to simple Rolfe and in the (1985). However, (1985) Schram (1990) flat pore fields level with surrounding cara- this cuticle. Intermediates of these rejected possibility, perhaps too precipitously pace principal types since Schram al. illustrated have less keels with reticulated et (1999: plate 3, fig. 7) may pronounced a the of cuticular that surface the fields. One so-called presence elongate structures comparable to pore evocative of lattice distinct theco- terminal were organs, a pore is often associated with each lattice found the of A rim of cuticle sometimes delimits the lat- stracan apomorphy on carapace some organ. free of thecostracan larvae tice et al. al. swimming stages (Schram organs (Jensen 1994a; Jensen et 1994b). et al. Often are minute structures of 5-10 1999). they pm length and 1-2 width but dimensions with pm vary spe- cies rather and some ‘gigantic’ lattice organs ap- 50 10 width the Lattice organs: a review proach pm length or pm (including in of the surrounding rim) some larger lepadid spe- Thecostracan lattice first described cies et al. organs were by (Jensen 1994a). New evidence Elfimov (1986), Their status as synapomorphies for demonstrates that the lattice or- derived setae et in the Cirripcdia and the Ascothoracida (in the latter gans are (Rybakov al. press) with function group they were initially dubbed cardiac organs, presumably a chemosensory (Hoeg but al. Jensen et al. (1994a) later identified them as et 1998). Dendrites from sensory cells project lattice of into the cuticle and terminate in to the organs on grounds homology) was firmly proximity The terminal the established by Ito & Grygier (1990) and Jensen et pores. larger pore seems most likely for molecules to reach the ele- al. (1994a). Lattice organs have been found in the pathway sensory cyprid larvae of thoracican, acrothoracican, and ments in Facetotecta and Ascothoracida. In Thora- cica and the rhizocephalan Cirripedia as well as in larval stages Rhizocephala, many small pores of the field facilitate ofAscothoracida (Jensen et al. 1994a) andrecently pore may diffusion towards the elements. These reside in the also in Y-larval Facetotecta (Hoeg & Kolbasov sensory pores exo- 2002). The extant crustacean parasite Tantulocarida cuticle with the thin epicuticle coating the walls well and the bottom of In as as the Cambrian Bredocaris have been the pores. Acrothoracica, minute forward as sister of the the but not the brought potential groups pores penetrate epicuticle underly- exocuticle. The of Thecostraca (Walossek et al. 1996; Walossek & ing possession numerous pores Muller but both lack lat- be for the lattice 1998), groups definitely may autapomorphic cirripedian tice within the Thecostraca the et al. & Kolbasov organs. However, organs (Hoeg 1998; Hoeg 2002). number, position and anatomy of lattice organs constitute systematically valuable characters (Hoeg & Kolbasov 2002). Materials and methods the lattice In thecostracans, organ complex nor- mally forms characteristic constellations of two Four Cretaceous thylacocephalan species were examined: Proto- zoeahilgendorfi Dames, 1886; P. damesi 1946; Pseude- clusters containing all together five pairs of lattice Roger, richthus cretaceus Dames, 1886, and Thylacocephalus cymolopos organs. Typically, the anterior cluster contains two Lange et al. (2001). while the includes three pairs posterior usually pairs. Series of possible lattice organs were found in eight speci- some have a reduced number of However, species mens of; Protozoea hilgendorfi: [BM: J 469-1, J 622-1; OR Contributions to Zoology, 71 (4) - 2002 161 Table 1. Information on fossils well series of lattice like thylacocephalan possessing preserved organs including length (* carapace without rostrum and in of - length posterior spines, mm); body type (**according to Schram et ah, 1999); position organs (ant anterior, mid - mid dorsal, number of and distance between post posterior); organs neighboring organs (in mm). P. hilgendorft Carapace length Body type** Position of series Number of llo’s Average distance mm (estimated*) along midline in each series between illo’s BM J469-1 (40) globose ant 3 0.5 mid 10 0.55 most 7 0.85 BM J622-I 78 elongate ant 12 0,5 mid 7 0.85 post 11 1.5 BM OR 59245 (2)-l 44 elongate mid ~20 0.5 BM OR 59245 (3)-l (50) intermediate postpost 1212 0.82 BM OR 59554 mid - 15-20 (3) (32) globose mid post 15-20 0.40 '-fc* -1 0.5OLn MB A 693.3 (24) globose mid ~13 0.6 MSNM i!2172il2172 42 intermediateintermediate mid 5 0.8 MSNM i20556120556 (30) globose post 4-5 0,7 P. damesi BM OR 59672 15 (4) 8 0.20.2-0.3- 0.3 post , 59245 (2)-l, (3)-1, OR59554 (3). MB: A 693.3. MSNM: il2172, Results 120556] and one specimen of P. damesi [BM: OR 59672 (4)] (see Table 1). Presence and position ofpossible lattice Similar structures occurred in organs sporadically some four speci- mens ofP. hilgendorfi: [BM: PI IC 82; In 42507a; OR 59672 and an additional Our lattice (16)] specimen MSNM: i7586-3, which is of putative organs were best observed in uncertain thylacocephalan affinity. P. of this hilgendorfi. Eight specimens species pre- Further possible lattice were noted, but their organs poor served series of lattice while potential organs, only preservation prevents unequivocal identification:P. hilgendorfi-. one P. damesi preserved such a series. Scattered