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Schram, F. R. (Dept. of Zool., Eastern Ill. U., Charleston, lll. 61920) 1974. Convergences between Late Paleozoic and modern caridoid Malacostraca. Syst. Zool. 23:323-332.-Study of the Middle Pennsylvanian, Mazon Creek, Essex fauna Crustacea (Johnson and Richardson, 1966; and Richardson and Johnson, 1971) of northeastern Illinois has prompted an examination of the entire Late Paleozoic radiation of caridoid malacostracans. Members of this radiation bear several gross anatomical similarities to Mesozoic-Cenozoic eucarid forms, yet are totally and completely distinct taxonomically from them. This convergent phenomenon affords a rare look at the evolution of a major taxonomic category extending over a period of some 300 million years. The comparison also bears on general theoretical considerations of diversity and convergence in evolutionary theory. [Convergence; Malacostraca; Diversity models.] The malacostracan crustaceans have been present in the fossil record since Cambrian time. The earliest and more primitive forms have been the Phyllocarida, forms with seven segments in their abdomens. In Devonian time a more advanced array of malacostracans apparently evolved from the phyllocarids. These forms, the Hoplocarida and the Eumalacostraca, are characterized by having six segments in the abdomen and a great variety of tagmatic and appendicular specializations that enabled them to more effectively dominate their environments. The Late Paleozoic radiation of advanced malacostracans was composed of hoplocaridans, syncaridans, and primitive eumalacostracans variously termed Eocarida (Brooks, 1962), caridoids, or schizopods. The Hoplocarida almost completely died out at the close of the Paleozoic except for the specialized Stomatopoda or "mantis shrimp." The caridoids persisted and formed the basis of the modern malacostracan radiation. The caridoids are all characterized by an array of features first elucidated by Calman (1909): two flagella on the first antenna, two segments in the thoracic protopod, five segments in the thoracic endopod, annulate and natatory thoracic exopods, thoracic epipodite gills, thorax and abdomien subequal, thorax containing viscera and covered by a carapace, abdomen developed with a special musculature for the distinctive backward-swimming caridoid escape reaction. These primitive caridoids gave rise to the groups which are the dominant malacostracans today, the Eucarida and Peracarida. The Peracarids have developed a brooding habit for protecting the young, and morphologically have a tendency to lose the carapace and greatly modify the body regions so as to diverge markedly from the caridoid morphotype. The evolution of the peracarids proper must remain the subject of another paper. The Eucarida have more closely retained the caridoid facies and are the dominant caridoid types today. The only living peracarids that can be termed "caridoid" are the Mysidacea. PERIPATETIC TAXONOMY The taxonomic affiliations of the Late Paleozoic caridoid malacostracans have been realigned several times. Huxley (1857) termed Pygocephalus cooperi an early form of decapod (Brooks, 1969, has since assigned this species to the genus Crangopsis). Huxley's designation influenced subsequent workers to categorize their fossil species as "macrurous decapods" as in Etheridge's (1877) description of some specimens of Anthrapalaemon and Wood

The relation of the Leptostraca to other groups of Crustacea has long been a problem of interest, although the alliance with the Eumalacostraca has been amply justified (Claus, 1872, 1888, Calm an, 1909). Claus discussed the essentially Malacostracan form of the appendages of Nebalia , and the mode of feeding by the aid of these appendages has been shown (Cannon, 1927) to be a specialized modification of the type shown by the simpler Malacostraca (Cannon and Manton, 1927, Some of the apparent differences between the Leptostraca and the Eumalacostraca have recently been shown to be differences of degree rather than of kind. Thus the seventh abdominal segment of Nebalia is found also in the embryo mysid, but is partially or completely fused with the sixth segment in the adult Lophogaster and Hemimysis respectively (Ma n to n, 1928,a and b). Thus the basal number of abdominal segments in the Eumalacostraca as well as the Leptostraca may be seven. The presence of a large caudal furca in Nebalia may also be a difference of degree, if this furca is homologous with the embryonic furca of a mysid (Manton, 1928, a). Other differences, such as the presence of a fully formed carapace adductor muscle, of cephalic liver lobes, etc., require further investigation. Of the resemblances between the Leptostraca and Eumalacostraca, the mode of development of the former is said to resemble that of the Mysidacea, but the embryology of no Leptostracan has been adequately followed, and the recent work on mysid development has considerably modified many previous views on this subject.

[Les fossiles paleozoiques recemment decouverts ont elargi nos connaissances sur l'histoire du super-ordre des Hoplocarida. Cette comprehension plus etendue accentue que les hoplocarides sont uniques et suggere qu'ils derivent independamment des leptostracees, separement et distinctement des eumalacostracees caridoides. L'auteur presente des raisonnements pour et contre l'origine multiple des eumalacostracees, avec une exposition abregee des interpretations polyphyletiques et mono-phyletiques de l'histoire eumalacostraceenne telle qu'on la comprend actuellement., Les fossiles paleozoiques recemment decouverts ont elargi nos connaissances sur l'histoire du super-ordre des Hoplocarida. Cette comprehension plus etendue accentue que les hoplocarides sont uniques et suggere qu'ils derivent independamment des leptostracees, separement et distinctement des eumalacostracees caridoides. L'auteur presente des raisonnements pour et contre l'origine multiple des eumalacostracees, avec une exposition abregee des interpretations polyphyletiques et mono-phyletiques de l'histoire eumalacostraceenne telle qu'on la comprend actuellement.]

Summary. 1 In Anaspides the statocyst is situated in the antennulary basal segment; the sac is boot-shaped, with a narrow slit-like opening antero-medially; about 35 jointed club-shaped hairs are suspended in a single series from the roof and near the opening of the sac; each hair contains one to two small bodies, presumably the statoliths. 2 Tattersall's discovery of what is probably a very rudimentary form of antennulary statocyst, without static hairs or statolith, has been confirmed in Hansenomysis. 3 No trace of antennulary statocyst has been found in Cumacea, Tanaidacea, Euphausiacea, Isopoda, and Amphipoda; although tactile hairs are often present on the basal segment, they do not represent the rudimentary static hairs, since they are usually present in Macrura (e. g., Crangon, Caridina), in addition to the statocyst. 4 The cephalic statocysts of Gamrnarus chevreuxi have been described. 5 In the Macrura examined there is a definite relationship between the stylocerite and the statocyst. When the inner fold of the stylocerite fully or partially overlies the basal segment, a well developed statocyst, as a rule, is present; when the stylocerite is lateral to the segment a comparatively simple statocyst (e. g., Crangon) or a very rudimentary form (e. g., Caridina) may be present, but generally it is absent (e. g., Hippolyte). Thus, within the Macrwa, the evolution of the statocysts is correlated with the character of the stylocerite. 6 In Palzmonetes and Leander the statocyst ia highly developed; the sac is peach-like, with a dorsal triangular opening overlapped by the stylocerite; the sensory cushion is on the floor of the sac, its longer axis being parallel (Palzmonetes) or transverse (Leander) to that of the sac; there are 56 typical plumose static hairs in Palzmonetes, 85 in Leander. The statoliths are of the usual type, namely, grains of sand taken in after each moult. 7 In Alpheus the sac is large, with an oval aperture directed laterally and partially overlapped by the stylocerite; a series of eight bristles overlap and protect an emargination in the dorsal wall of the opening; the sensory cushion is inconspicuous, on the almost vertical floor of the sac; there are about 65 typical static hairs. 8 Besides the static hairs, the statocyst of Alpheus has a great number of tactile hairs scattered on the outside of the sensory cushion. These may possibly be folded into the cavity during the formation of the statocyst. 9 The statocyst of Ogyrides is almost identical with that of Alphew, and supports the view that this genus belongs to the family Alpheida. 10 In Crangon vulgapis the stylocerite is lateral to the statocyst, the opening of which is protected by a row of 25 to 27 large bristles; the 30 static hairs axe arranged in series on an undulating sensory ridge on the lateral wall of the sac. 11 The most rudimentary form of statocyst that has hitherto been known to occw in Decapoda is present in Caridina and Xiphocaris. It is just an elongate shallow depression on the dorsal surface of the antennulary basal segment, and contains neither static hairs nor statoliths. 12 I have not dealt with the detailed histology of the muscular and nervous systems, although they are figured in detail. For full description reference should be made to Prentiss (1901). The sensory ganglion cells are all typically bipolar and elongate. They are situated at some distance from the bases of the static hairs, each of which they supply with a single fibre from a single cell (the peripheral fibre enters only the spherical enlargement of the hair-base). They may be regarded as equivalent to neurons between the peripheral and central fibres (PL II. fig. 12: PL III. figs. 25, 27; P1. IV. fig. 31; PI. V. figs. 36, 37; P1. IX. fig. 62).

The structure of the thoracic epipodites on the stomatopod Hemisquilla ensigera (Owen) suggests that the primary function of these structures is respiratory. The presence of respiratory structures on the thoracic somites, coupled with other already published evidence links the hoplocarids and the other eumalacostracans to a common phyllocarid ancestor and argues against a polyphyletic origin for the Eumalacostraca.

Siewing (1963) suggested that hoplocarids occupy an unusual phylogenetic position within the Eumalacostraca in that they have lost the first abdominal segment. In most other eumalacostracans the pretelsonic abdominal segment was lost from the original phylocaridan-type abdomen of seven segments. The major argument presented supporting a loss of the first abdominal segment in the Hoplocarida is the presence of an extra pair of lateral arteries in the first abdominal segment. The usual condition is one pair of lateral vessels per segment. Accounts of the circulatory system of adult stomatopods appear to concern only the genus S quilla (see Komai & Tung, 1931 ; Mayrat, 1958). My observations on the circulatory system of Hemisquilla ensigera (Owen) (see Stephenson, 1967) have shown significant differences from S quilla on the disposition of the lateral vessels.