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Results
The cladogram (Fig. 8) reveals a far from random array of species. The out-group, Mictocaris, Bogidiella and the two species of Pseudingolfiella, remained apart from the in-group whether or not we enforced an out-group constraint in PAUP. We have placed capital letters to focus attention on the most interesting branches of the cladogram. These branches display good apomorphies, i.e., with high consistency indexes. For instance, the lack of an ischial lobe (character 14) on the maxilliped in Mictocaris halope results in a strong emphasis by the program on the presence of this lobe in Bogidiella and Pseudingolfiella. This example is also illustrative of how such changes must be interpreted in a relative way. The mictaceans are not necessarily a close sister group to the bogidiellids and pseudingolfiellids (node A). We lack in fact countless numbers of species relevant to amphipod history that never made it through to the present, and we undoubtedly lack many deep groundwater forms that could enlighten us on the origins and early evolution of the group. We merely wish to say, before we go on to interpret character state changes, that given these taxa and these particular characters, and provided one watches out for specific assumptions, then the particular phylogenetic patterns we obtained seem to prevail.
At the base of the ingolfiellidean clade, we note Metaingolfiella, characterized by the uniquely partially fused second thoracic segment into the head, and the fusion of elements in uropod 1 rami and uropod 3 peduncles. (This last feature is interesting in regard to the caudal furca/rami situation in other crustaceans, Schram & Koenemann, in lit.). Nevertheless, these autapomorphies are associated with many plesiomorphic features that insures this species remains at the base of the ingolfiellidean clade.
Stygobarnardia (node B) regains a vestigial mandibular palp in this scenario. It was lost in the transition from the out-group to the in-group (node A). While such character reversals appear to pose problems for diagnosing taxa, they can perhaps best be understood in terms of paedomorphic shifts of developmental timing of events.
The “trogloleleupians” emerge as paraphyletic. The members of a small clade (node C), made up of T. eggerti, T. dracospiritus and T. leleupi, share weakly developed lenticular organs, while the other two “trogloleleupian” species are set off by having either strongly developed lenticular organs, or a differently implanted palmar angle seta. Two new genera are necessary to recognize their isolated locations on the tree.
Ingolfiella is retained as a distinct genus despite its wide habitat and biogeographic spread. The shortening of the uropod 1 stands out as an apomorphy. This character state is stable throughout the genus, no alternating of states is encountered. Other changes defining the Ingolfiella node are the transitions from blades to spines on the margin of the dactylus in gnathopod 2, and the diminishing number of crenelations on the palmar margin (although reversals to the absent state are seen, scattered throughout the Ingolfiella clade).
A coastal Mediterranean clade (Fig. 8, node D) is separated on the basis of losing one seta on the inner lobe of maxilla 1. These setae are sparse relative to what is common in gammaridean amphipods. Reverting from four to three setae would seem to reduce the power of that limb for handling food particles.
The transition to the brackish water grade (Fig. 8, between nodes E and F) is defined by a single character state shift: the size of gnathopod 2 going from smaller than the length of pereiopod 3, to subequal to this length.
Finally, a marine clade can be recognized (node F) on the basis of gaining one tooth/spine on the dactylar margin of gnathopod 2, but reversals are possible further up in the clade. Furthermore, a process on the tip of the propod reverts to an unmodified state on this node but is reversed again in the Australian species, the South-African, the Atlantic Ocean, and a Caribbean species.
We have omitted from our analysis four species: Ingolfiella abyssi, I. atlantisi, I. littoralis and I. kapuri. The many unscoreable character states for these species clouded the initial analyses that we ran. When we removed one species after the other from the heuristic searches, the number of trees became fewer and the branching pattern of the remaining taxa began to stabilize. We would hope that some day additional material of these taxa will become available that will allow more characters to be scored for these species. Furthermore, it is our desire that by laying out the characters and their alternative possible states in the detail that we have, that future describers of ingolfiellidean species will become more focussed on what descriptors they should seek to elicit from the study of their specimens than has been the case up to this point.