The position of the possible lattice organs close to the thylacocephalan dorsal midline resembles somewhat that of the lattice organs in the thecostracan Crustacea. However, the characteristic 2 + 3 arrangement found in the latter was not observed in the examined Thylacocephala.
In a morphological comparison, it is immediately obvious that the reconstructed possible lattice organ described above does not correspond completely to any of the principal types of lattice organs occurring in the extant thecostracan crustaceans as described by Jensen et al. (1994a) and Høeg & Kolbasov (2002). The possible lattice organs are very different from the ‘pore field’ type with the characteristic perforated surface (Fig. 6A). Such pore fields are autapomorphic for the thoracican and rhizocephalan thecostracans (Høeg & Kolbasov 2002). However, thecostracan lattice organs of the second principal type do have a smooth cuticle seemingly ‘folded’ into one elongated keel that sits in a depression (Fig. 6B). This ‘keel in a trough’ type is considered more plesiomorphic within the Thecostraca, occurring in the Facetotecta (Høeg & Kolbasov 2002) as well as in the Ascothoracida and the Acrothoracica (Jensen et al. 1994a). Lattice organs from both Acrothoracica and Facetotecta are more shallow with less pronounced keels than depicted in Fig. 6B (Jensen et al. 1994a; Høeg & Kolbasov 2002). The possible lattice organ of the Thylacocephala resembles the plesiomorphic lattice organ type in having the same basic outline, i. e., elongated depression of a smooth cuticle with a tendency to display folds, as observed in its broader anterior region. These folds could represent the keel, but they do not present the same tight fitting keel sitting in a narrow depression. The ridges that surround the possible lattice organs may be comparable to the elevated rims surrounding the central area of lattice organs reported from some thecostracans. The lack of definitive evidence of terminal pores in the thylacocephalans is troubling. The smooth ‘keel in a trough’ type lattice organ always possess a terminal pore, whereas the terminal pore is absent from the apomorphic pore fields in some species (Jensen et al. 1994a). However, the lowered areas anterior and posterior to the possible lattice organs (Fig. 2) again bear resemblance to lattice organs from some thoracican thecostracans. Here the lattice organ includes an anterior concavity housing the terminal pore as well as a slight posterior concavity (Jensen et al. 1994a).
Fig 6. Examples of thecostracan lattice organs. A, ‘Keel in a trough’ type from Ascothoracida. B, ‘Pore field’ type from Thoracica (both from Høeg & Kolbasov, 2002) tp, terminal pore.
In general the possible lattice organs of the thylacocephalans are several times larger than the lattice organs of thecostracans. Perhaps this larger size could be viewed as an allometric consequence of a much larger body size. While thecostracan larval stages with lattice organs measure around 1 mm (Jensen et al. 1994a; Høeg & Kolbasov 2002), the examined thylacocephalan fossils ranged from a few to eight cm to say nothing of other thylacocephalans; some like Dollocaris and Ostenocaris Arduini et al. 1982 surpass 20 cm. In proportion to total body length, however, thecostracan lattice organs actually appear slightly larger than those of the thylacocephalans. Since lattice organs are evidently derived setae (Rybakov et al. in press) it is interesting to notice that chemosensory setae may reach a length of at least 2.0 mm in specimens of the decapod crustacean Homarus americanus (Milne-Edwards) with body sizes comparable to the protozoean thylacocephalans (Derby 1982).
On the other hand, the number of observed possible lattice organs clearly exceeds that of lattice organs in the Thecostraca, even if the ‘extra’ pore fields observed in some thecostracans (Jensen at al. 1994a) may reduce this difference. The different number of observed organs could depend on body size too, but may also be attributed to the fact that only certain small larval stages in thecostracans are equipped with lattice organs, while it is likely, though not proven, that the observed possible lattice organs occur in mature specimens of thylacocephalans.
Because of these morphological similarities, we might assume that the large and numerous possible lattice organs in Thylacocephala are really homologous to the lattice organs in the Thecostraca. This can argue for a relationship between the two groups. However, it can never serve to include the thylacocephalans within the Thecostraca sensu Grygier (1987) primarily because the highly conserved apomorphic pattern of 2 + 3 lattice organs in Thecostraca is absent in the Thylacocephala.
If a sister group relationship is accepted, an opportunity to compare the body plan of the Thecostraca and the emerging information concerning the body plan of the Thylacocephala presents itself. Thecostracans have a comparatively short trunk consisting of no more than 11 segments including up to seven in the thorax. Such short bodies also occur in other crustaceans, which are often placed within the Maxillopoda (Schram 1986, but see also Schram & Hof 1998). Because most of the adult thecostracans are highly modified for sessile or parasitic lifestyles, only few lend themselves to such a direct comparison. The thecostracan ascothoracid genus Synagoga Norman, 1888 has seven thoracic somites and four abdominal in addition to a typical crustacean cephalic region of five segments (Grygier 1983; 1984). The posterior end of a thylacocephalan body includes a battery of eight (sometimes even more) limb-bearing segments. This does not accord with any of the thecostracans. Furthermore, comparison with the wider array of maxillipodans presents similar problems. To force thylacocephalans into these more inclusive groups would require a postulated combination of elements that is difficult to envision at this time, viz., the fusing of two short tagmata to form in some way the 8+ segments of the posterior thylacocephalan body region. At present, the Thylacocephala cannot convincingly dispute the potential sister group relationship between Thecostraca and the Tantulocarida as well as Bredocaris (Walossek & Müller 1998).
Attempting to see thylacocephalans in an ancestral role with respects to thecostracans also demands large hypothetical rearrangements in the body plan. Presently, it appears that the thylacocephalans may have had at least three pairs of limbs between the mandible and the posterior limb battery (Polz 1993, Schram 1990, Lange et al. 2001). This suggests that thylacocephalans had a maxilliped, but speculation on homology to the maxillipeds in Thecostraca seems premature. Maxillipeds do occur in some maxillopodans, but together with a correspondingly reduced number of remaining thoracic limbs (Grygier 1983, Schram 1986). Again, the number of remaining limbs would seem too low when compared to the 8+ battery in thylacocephalans.
Finally, though the oblong structures (Fig. 1D) and the more dorsal possible lattice organs in P. hilgendorfi are similar, it is more interesting that the former are also similar to the more ventral and elongate structures on the carapace of Dollocaris (Fig. 5). Yet another elongate and slightly curved structure has been reported in a presumed lateral position on the carapace of the Silurian marine fossil arthropods, Dictyocaris Salter 1860 (see Van den Brugghen: 1995, fig 1a). Dictyocaris reached the formidable length of at least 30 cm, but because corresponding limbs, eyes, gills etc. are presently unknown for this taxon, the phylogenetic position of these mysterious arthropod carapaces remains elusive (Van den Brugghen, 1995; Rolfe 1969). The identification of such, elongated, carapace structures constitutes a first indication of a probable thylacocephalan affinity for Dictyocaris. This begs for a new investigation of Dictyocaris especially in regard to cuticular structures.
In summary, this is the first time that lattice, or possible lattice, organs have been found outside the Thecostraca sensu Grygier. However, the structural and topographic differences between possible lattice organs and lattice organs may eventually tell us more about the plesiomorphic condition in a possible thecostracan out-group than posit any close phylogenetic relationship. In addition, this harkens back to the original description and interpretation of the thylacocephalan Ostenocaris cypriformis (Arduini et al. 1980), which postulated a barnacle affinity in the context of thylacocephalans representing ‘the big cyprid’. We might ask, has the history of this group come full circle?