850


Journal of Paleontology 91(4):847–857


Repositories and institutional abbreviations.—Unpublished serial acetate peels reposited in the Naturalis Biodiversity Center in Leiden, Netherlands, were utilized for this study (Breimer and van Egmond, 1968). Raw scanned peel data are available in Supplemental Data 3–8.


Results and discussion


Hydrospire morphology.—There have been six models generated thus far: Monoschizoblastus rofei (Etheridge and Carpenter, 1882) (Fig. 4.13–4.15), Ellipticoblastus ellipticus (Sowerby, 1825) (Fig. 4.10–4.12), Diploblastus glaber (Meek and Worthen, 1869) (Fig. 4.7–4.9), Deltoblastus permicus (Wanner, 1910) (Fig. 4.4–4.6), Cryptoblastus melo (Owen and Shumard, 1850) (Fig. 4.16–4.18), and Pentremites godoni (DeFrance, 1819) (Fig. 4.1–4.3). All of these taxa have spiraculate morphologies and represent the late Paleozoic spiraculate gross body plan. Examination and description of hydrospire structure from the completed models show them to be character-rich and allow the identification of several novel characters. The number of hydrospire folds has previously been used to delineate taxa, and by including this character in the analysis, it will be possible to test the validity of using hydro- spire count to erect taxa. The number of hydrospire folds in each group (i.e., the series


of folds that form a single respiratory structure) varies between the models. The numerous species of Pentremites vary in the number of hydrospires per group, and this number can vary between individuals of the same species and ontogenetically (Macurda, 1967; Macurda and Breimer, 1977; Dexter et al., 2009), although this is exceptional. In most taxa with one or two folds, the number is consistent among individuals. However, some taxa have fewer hydrospire folds on the anal side, likely providing additional space for associated structures such as the gonads and/or anus. This can be seen in two of the six models (Fig. 4.1–4.6). In D. permicus, for example, hydrospire folds are paired in each group except for those within the CD interray (the anal side), where single folds are present (Fig. 4.4–4.6). This reduction is also seen in P. godoni,where theanalsidehas four folds per group, whereas other groups all contain five folds (Fig. 4.1–4.3). This reduction is not seen in either E. ellipticus or M. rofei, and these taxa have a single fold per group whereas D. glaber and C. melo have two folds per group. Variation of hydrospire morphology suggests their utility to


differentiate taxa. Two of the six completed models, E. ellipticus and M. rofei, are within the traditionally described family, Orbitremitidae, but show variable hydrospire morphology (Fig. 4.10–4.15). Ellipticoblastus ellipticus (Fig. 4.10–4.12) has hydrospire fold pairs that begin nearly the same distance apart as


those of M. rofei (Fig. 4.13–4.15) but remain closer together as they extend vertically toward the spiracles. The paired hydrospire folds of M. rofei bow outward slightly prior to tapering nearer to the spiracle openings (Fig. 4.13). The number of hydrospire folds in each group also varies between families. Diploblastus glaber (Fig. 4.7–4.8) and D. permicus (Fig. 4.4–4.5) show two folds within each group, whereas both E. ellipticus and M. rofei have a single fold per group. In addition, the surface area of the fold is variable between


the generated models. Deltoblastus permicus (Fig. 4.4–4.6), M. rofei (Fig. 4.13–4.15), and C. melo (Fig. 4.16–4.18) all have folds that extend shallowly into the coelomic cavity compared to E. ellipticus (Fig. 4.10–4.12) and D. glaber (Fig. 4.7–4.9), both of which extend further into the coelomic cavity. Rather than increasing the extent of the folds, P. godoni (Fig. 4.1–4.3) has additional narrow folds to increase the surface area. The variation in surface area is likely directly related to gaseous exchange between the hydrospires and the coelomic cavity (Dexter et al., 2009). The hydrospire cleft (Fig. 2) is also variable among these species and may be related to changing the surface area of the fold. Monoschizoblastus rofei possesses a long, thin cleft (Fig. 4.14), whereas D. glaber has a short, stout cleft (Fig. 4.9). Pentremites godoni has an elongate cleft to accommodate the additional folds present at each pore. Notable variation exists for the ratio of hydrospire pores to


hydrospire folds to spiracular openings. In M. rofei, there is a single fold per pore, and each of these folds extends through the theca and is expressed as an individual spiracle at the summit (Fig. 4.13, 4.14). Conversely, in P. godoni, there are five folds per pore that merge into a single tube that extends toward the summit. Finally, this tube merges with an adjacent tube to be expressed as a spiracle at the summit (Fig. 4.14). Although only six models were generated for this study, all


of the spiraculate morphotype, it is clear there is significant variation both between and within previously described families. Additional models of all morphotypes will result in an increased understanding of variation and similarities between hydrospire structures.


Blastoid phylogeny.—The morphology described in the preceding provides a baseline to evaluate internal character data for blastoids. Preferably, all of the taxa used to infer blastoid phylogeny would be represented by species for which there are both specimens to code externalmorphology and peel data to code internal morphology. As there were only a few taxa (nine) in this analysis, character data had to be reduced to examine the relationships between these taxa. This was done by examining all character data as a whole and determining characters that were constant and uninformative among the taxa. The uninformative


Figure 4. (1, 2) Anatomical model of respiratory structures of Pentremites godoni (DeFrance, 1819) in (1) oblique lateral and (2) aerial views. (3) Representative section of P. godoni showing the abundance of folds, elongate cleft, and plate boundaries. (4, 5) Anatomical model of respiratory structures of Deltoblastus permicus (Wanner, 1911) in (4) oblique lateral and (5) aerial views; note the reduction of hydrospire folds in the anal area. (6) Representative section of D. permicus showing the petite hydrospires and thick plates. (7, 8) Anatomical model of respiratory structures of Diploblastus glaber (Meek and Worthen, 1869) in (7) oblique lateral and (8) aerial views. (9) Representative section of D. glaber showing paired folds in each group and a stout hydrospire cleft. (10, 11) Anatomical model of respiratory structures of Ellitpicoblastus ellipticus (Sowerby, 1825) in (10) oblique lateral and (11) aerial views. (12) Representative section of E. ellipticus showing the long thin hydrospire cleft of each hydrospire fold. (13, 14) Anatomical model of respiratory structures of Monoschizoblastus rofei (Etheridge and Carpenter, 1882) in (13) oblique lateral and (14) aerial views. (15) Representative section of M. rofei exhibiting single folds per group. (16, 17) Anatomical model of Cryptoblastus melo (Owen and Shumard, 1850) in (16) oblique lateral and (17) aerial views. (18) Representative section of C. melo exhibiting short bifurcating hydrospire clefts, circular hydrospire ducts, and clear plate boundaries. All scale bars = 5mm.


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