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Journal of Paleontology 91(4):847–857


external openings at the end of completely internal hydrospire folds (Waters, 1988; Fig. 1). Thesemorphological groups have been examined separately


on several occasions over the past 50 years (Breimer and Macurda, 1972; Macurda, 1983; Breimer, 1988a, b; Waters and Horowitz, 1993), but few studies have utilized rigorous phylogenetic methodologies to evaluate evolutionary relation- ships (Bodenbender, 1995; Bodenbender and Fisher, 2001). The results of a recent phylogenetic analysis by Atwood (2013) suggested that spiraculates are polyphyletic and nested within a larger fissiculate clade, agreeing with previous studies (Waters, 1990;Waters and Horowitz, 1993). In addition, several blastoids, such as Pentremoblastus and Conuloblastus, appear to be transitions between the fissiculate and spiraculate morphotypes. These genera have hydrospire slits that lead to bean-shaped or underdeveloped spiracles or have well-developed spiracles and hydrospire slits only partially covered by ambulacral side plates.


Hydrospire morphology


Respiratory structures of extinct blastozoan echinoderms are diverse, highly variable, and often clade defining (Paul, 1968, 1972; Sprinkle, 1973; Schmidtling andMarshall, 2010).The pores and associated structures of many blastozoans have been examined (Paul, 1968, 1972), but the explicit study of blastoid respiratory structures is lacking. Many studies (not limited to Breimer and Macurda, 1965; Macurda, 1967, 1969, 1975; Breimer and Joysey, 1968; Breimer et al., 1968; Breimer, 1970; Breimer and Dop, 1975; Macurda and Breimer, 1977) incorpo- rated a thorough report of hydrospire structure into systematic descriptions, but few studies (Beaver, 1967; Dexter et al., 2009; Schmidtling and Marshall, 2010; Huynh et al., 2015) primarily discuss function or efficiency of these structures. The respiratory structures of blastoids (i.e., hydrospires)were


The incurrent pores lead to hydrospire folds (ranging from one to 10 in number; Fig. 2) and finally to the excurrent openings, spiracles, at the summit (Sprinkle, 1973; Waters et al., 2017). Hydrospire morphology and terminology can be confusing,


specifically with the variation with fold number. Terminology herein follows the morphology outlined in Beaver (1967). In spiraculates, hydrospire folds occur at pores (Fig. 2.1–2.3) that are visible on the exterior of the organism. The pore leads to a hydrospire cleft, which is the portion of the fold between the pore and the final termination at the hydrospire tube (Fig. 2.1–2.3; hydrospire tube is synonymous with hydrospire canal in Schmidtling and Marshall, 2010). Some hydrospire clefts may bifurcate early (Fig. 2.2), whereas others are elongate and rest upon plates to accommodate additional folds (Fig. 2.3). At a given pore, multiple folds can be grouped to form hydrospire groups (Fig. 2.2, 2.3).The hydrospire tube is the expanded terminus of the fold that eventually leads to the spiracle opening at the top of the theca.Depending on the genus, this tube may reach the summit as a single spiracle or it may combine with adjacent tubes prior to reaching the summit. Previous interpretations of these structures have either


suggested that hydrospire walls were: (1) open meshworks that allowed for gaseous exchange between the coelomic fluids and ambient seawater (Macurda, 1973; Beaver, 1996) or (2) consisting of tiny calcite crystals (Beaver, 1967). Most workers assumed that the hydrospire walls were permeable, but the nature of wall preservation leaves little support for permeable folds (Beaver, 1996). The orientation of the section (perpendicular or oblique to the center axis of the blastoid) determines whether the more complex hydrospire meshwork is uncovered (Beaver, 1996). Macurda (1973) and Beaver (1996) provided evidence on the nature of the stereomic microstructure of blastoids as composed of a meshwork similar to that of modern echinoderms. The external expression of hydrospires forms the basis of


lightly calcified, porous, and fold-like internally (Beaver, 1967; Sprinkle, 1973). The two main morphotypes, fissiculate and spir- aculate, are different both externally and internally. Fissiculates possess hydrospire slits, which are open to the exterior along the length of the hydrospire fold but are either covered by side plates or exposed above them and cross the deltoid-radial suture (Fig. 1.2). Spiraculates possess incurrent pores that line the ambulacra and are either positioned between the side plates or penetrate the adjacent radial and/or deltoid plate (Fig. 1.1).


1 2


differentiation between fissiculates and spiraculates (Beaver et al., 1967); however, the internal architecture of hydrospires has yet to be studied. Typically, hydrospire data are drawn and reported from one to several sections near the top or center of the theca (e.g., Breimer et al., 1968; Breimer, 1970; Breimer and Dop, 1975; Macurda and Breimer, 1977). This can provide information on general size and number of folds but not on changes in shape and proportion as they pass through the thecal interior. It is, therefore, critical that in-depth examination of these structures be performed to provide a basis for understanding similarities and differences among taxa so that these data can be included into subsequent phylogenetic analyses. Hydrospires, unlike other internal structures (such as the gut and reproductive organs), are constructed of thin calcareous walls (Beaver, 1967) and are typically preserved within the theca. As the hydrospires are internal organs, new visualization methodology had to be developed (Waters et al., 2014, 2015) to digitally render and manipulate complete hydrospire structures. Preliminary work (Waters et al., 2014, 2015; Bauer et al., 2015) suggests that hydrospires occur in a variety of forms and are likely important in delineating higher taxonomic groupings.


Figure 1. Generalized diagrams of the two primary blastoid morphotypes. (1) Spiraculate morphotype with incurrent hydrospire pores lining the ambulacra leading to four excurrent spiracles and one large anispiracle. (2) Fissiculate morphotype with four slits on each side of and parallel to the ambulacra crossing the radial-deltoid plate boundary.Modified from Beaver (1967).


Materials and methods


Paleozoic echinoderm workers have employed techniques such as producing thin sections or acetate peels to study internal


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