Waters et al.—Respiration in blastoid hydrospires
paleontology now allow for the reconstruction of hydrospires in three dimensions and the testing of fluid flow models through the respiratory system (Bauer et al., 2015, 2017).
Imaging the interior of blastoid thecae
Virtual paleontology studies fossils through digital visualization of the three-dimensional morphology (Sutton et al., 2013). The raw data for virtual 3D reconstructions of internal morphologies in fossils usually involve tomographs, a series of two-dimensional parallel slices, of a specimen that are gathered by either destructive or nondestructive methodologies (Cunningham et al., 2014). Historically, tomographic data consisted of thin sections or acetate peels (Sollas, 1904; Stensio, 1927; Muir-Wood, 1934; Stewart and Taylor, 1965) collected through destructive techniques such as serial grinding, sawing, or slicing. Until the advent of high-speed computing and 3D reconstruction software, individual tomographic slices provided the morphologic information required to taxonomic delineation. These tomographic slices were often coarsely and unequally spaced. Three-dimensional reconstructions were rare, time consuming, and accomplished via the construction of physical models (Sollas, 1904; Jarvik, 1954). Jefferies and Lewis (1973) and Schmidtling and Marshall (2010) offer examples of echinoderm reconstructions using serial sections. Acetate peels were widely used to interpret the internal morphology of brachiopods (e.g., Copper, 1967; Posenato, 1998; Schemm- Gregory, 2014), corals (e.g., Wang, 2013), and many other fossil groups, including blastoids (Breimer and Macurda, 1972). Modern nondestructive methodologies include X-ray tomo- graphy, magnetic resonance imaging, and neutron tomography (Cunningham et al., 2014). Modern techniques produce high-resolution 2D images that
can be reconstructed as three-dimensional computer models using appropriate visualization software. X-ray tomography is capable of micron-scale resolution, and synchrotron facilities are increasingly available; hence, the technique is commonly used in paleontology. X-ray tomography works well when the internal structures being imaged and the material filling the voids have significant density contrast (Abel et al., 2012) or when specimens are preserved as molds (Rahman et al., 2015a). These techniques are not as suc- cessful in specimens showing little density contrast between internal structures and the void filling because materials with similar densities attenuateX-rays to similar degrees, and so are not clearly differentiated in the resulting slice images. This situation commonly occurs in blastoids as well as other fossil echinoderm clades (Waters et al., 2015). Although digital techniques, such as synchrotron imaging,
are clearly preferable when available and can produce specta- cular results (Rahman et al., 2015b), acetate peels provide important morphological data critical to studies of blastoid phylogeny that are currently unavailable using nondestructive imaging technology, particularly where there is low-density contrast within specimens. Even in these cases, the boundaries separating echinoderm plate material and hydrospires from spary and micritic thecal fill are clearly defined in acetate peels because echinoderm plates are constructed of microporous stereom. Consequently,we have concluded that legacy collections
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of acetate peels remain a viable data source for visualizing the internal structures of blastoids.
Previous studies of water flow through blastoid hydrospires
Hydrospires are lightly calcified internal infoldings of the deltoid and radial plates of blastoids typically formed adjacent to the ambulacra. They can occur singly or in groups from two to 10 and are presumed to have had a respiratory function; they may be reduced in number or absent in the anal (CD) interray (Breimer and Macurda, 1972). Hydrospires have two closely spaced walls forming the hydrospire fold with a bulbous termination called the hydrospire tube (Beaver et al., 1967) or hydrospire bulb (Schmidtling andMarshall, 2010). In spiraculates, hydrospires are connected by hydrospire pores and canals located along the edges of the ambulacra and spiracles at the summit of the theca. Bauer et al., (2017) discusses hydrospire morphology, the history of study, and phylogenetic implications in greater detail. Schmidtling and Marshall (2010) were the first authors to
consider the flow of water through the hydrospire system by reconstructing a high-resolution, three-dimensional hydrospire model of the blastoid Pentremites rusticus Hambach, 1903 using serial sections. Drawing on their detailed morpho- logical analysis, they proposed the following flow of water through the system. Water enters the hydrospire through the hydrospire pores and passes through the pore canal to reach the hydrospire fold. No significant gas exchange happens in the hydrospire pore or pore canal. Water then enters the hydrospire fold, where gas exchange occurs. Oxygen-depleted water exits the hydrospire system via the hydrospire tube and spiracles. Using the principle of continuity, Schmidtling and
Marshall (2010) proposed a model of passive water flow through the hydrospire system. Water velocity would slow dramatically as water entered the hydrospire folds from the pores and pore canals. Because the hydrospires of Pentremites rusticus expanded in size adorally, water maintained a more or less constant velocity relative to incurrent velocity and exited the spiracle at a similar velocity unless the spiracular cover plates were used to reduce the size of the spiracular opening.
Using the observations and calculations summarized in the
preceding, Schmidtling and Marshall (2010) proposed a resi- dence time of water in the hydrospires of ~ 385 seconds. Their hypothesized water flow in Pentremites rusticus is on the same
order of magnitude as modern invertebrates with passive fluid flow but on the order of 1,000 times smaller than flow rates from organisms employing active ciliary-driven water flow. Huynh et al. (2015) expanded the observations and
hypotheses of Schmidtling and Marshall (2010) by constructing a highly enlarged (72x) physical 3D model of a partial hydro- spire and conducting fluid flow experiments. The results of their experiments seemed to confirm the hypothesis of Schmidtling
and Marshall (2010) that water entering the hydrospire pores flowed parallel to the pores while in the hydrospire fold without mixing with water entering adjacent pores. These water “units” did not mix until they reached the hydrospire bulb and began their exit from the system.
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