INTRODUCTION

The purpose of this study is to use characters visible from the exterior of biserial fenestrates to define morphospace occupied by observed taxonomic units (OTUs) assigned to diverse genera and to determine whether OTUs, assigned a priori to a given genus form a distinct cluster within a smaller region of that morphospace. If there is little or no clustering of OTUs assigned to a given genus within the inclusive morphospace, the reality of fenestellid genera based largely on internal characters might be more severely questioned, but distinct clusters of OTUs belonging to those genera supports their discrimination as morphologically characterizable subgroups based on measurable external morphological characteristics (Hageman 1991).

This is a study of genera. The existence of nominal species is acknowledged, but should be considered in the context of within genus variation. The concept of fenestrate species distributions in morphospace will be treated elsewhere.

Historical Summary

In general, the characterization of genera is a subjective enterprise. As more information accumulates, if morphological detail rather than gross morphology is considered more closely, some supposedly well-known genera begin to be reinterpreted as a complex of related but distinguishable genera. The brachiopod genus Productus is one of the better known examples, with 29 genera being named in a single volume based on type species originally described as a species of Productus (Muir-Wood and Cooper 1960). Fenestrate bryozoans similarly experienced proliferation of generic names during the late 20^th century as formerly widely familiar, easily discriminated genera were subdivided.

Fenestrate bryozoans are a major component of the mid- to late Paleozoic marine fossil record. With one known exception, they are characterized by a shared colonial growth habit. They consist of rigidly erect, narrow branches in which all autozooecia open toward one side of the branch (the same direction as neighboring branches), and the number of rows of autozooecial apertures along the branch is limited, varying from two to about ten. Branches typically are laterally linked by anastomosis, by skeletal bars (dissepiments) lacking autozooecia, or by short lateral branches bearing autozooecia . Branch proliferation occurred either by dichotomous division of the growing tip of the branch or by lateral branches (pinnae) that formed on the sides of the parent branch just behind the growing tip. These two types of branch multiplication were used from the late 19^th century through most of the 20^th century as the basis for the two major taxonomic subdivisions of the fenestrate bryozoan clade (e.g., Bassler 1953), although recently some bryozoan taxonomists have emphasized other morphological characters that result in the mixing of branching types within families (e.g., Morozova 2001). The focus of this study is biserial forms traditionally assigned to the family Fenestellidae.

Early Concepts of Fenestellid Genera. The first genus of fenestrate bryozoans to be characterized in print was Archimedes, named by David Dale Owen in 1838 who (p. 13) referred to it only as "...a fossil, described by Lesueur under the name of Archimedes, on account of its screw-like form.". The following year, Lonsdale (1839, p. 677) introduced the name Fenestella for, "A stony coral, fixed at the base and composed of branches, which unite by growth and form a cup. Externally the branches anastomose or regularly bifurcate; internally they form a network, the intervals being generally oval, one row of pores on the branches externally, the openings being circular and projecting when perfect. The branches, when regularly bifurcated, are connected by distant, transverse processes, in which no projecting pores are visible."

The name Archimedes was used in print for solid helical structures a few times between 1838 and 1857, and several species were named from around the world as Fenestella. In 1857 Hall (p. 177) recognized that Archimedes and Fenestella are related and summarized their differences: "In all essential characters, the foliate expansions of Archimedes corresponds to Fenestella, according to the extended description of this genus by Mr. Lonsdale; and in detached fragments cannot be distinguished generically from other forms of the same genus....The mode of growth, therefore, constitutes the only reliable character for separating Archimedes from Fenestella; and should this character be hereafter considered of sufficient importance, I propose to retain Le Sueur's original name 'Archimedes'.

John Phillips (1841) added a third genus, Hemitrypa, to the Paleozoic fenestrates soon after Archimedes and Fenestella were established. Phillips (1841, p. 27) characterized Hemitrypa as "...a cup-formed mass; external surface wholly covered with numerous round pores of cells...associated in double rows...The internal face was like that of some Fenestellæ, but the peculiarities of the external surface seem to require generic separation." The generic name Hemitrypa relates to Phillips' mistaken understanding that the fenestrules extended from the internal surface only halfway through the skeleton. He did not realize that the visible external surface was a superstructure consisting of a finer mesh than the branches, which was developed as an integrated canopy above the branches by proliferation and fusion of lateral processes at the outer ends of long spines extending from the surface of the underlying branches.

The first three genera of fenestrate bryozoans to be characterized were fenestellids in the broad sense, i.e., branches increased by bifurcation rather than developing as pinnae, were laterally linked, and each contained two rows of autozooecia. The differentiation of these three genera foreshadowed characterization of newly named "fenestellid" genera into the late 20^th century. That is, "fenestellid" genera were differentiated on the basis of gross colony morphology (Archimedes vs. Fenestella) or on the basis of meshwork characteristics and/or morphological characteristics visible on branch surfaces (Fenestella vs. Hemitrypa). There was a roughly parallel development in the characterization of non-pinnate fenestrate bryozoans with three or more rows of autozooecia per branch, but this paper addresses the biserial "fenestellids".

Generic Concepts That Include Internal Characters. Thin sections of fenestrate bryozoans were used extensively by Ulrich (1890) to supplement morphological information available from the surface of colonies, but he did not establish any new fenestrate genera based on thin sections. While not based on prepared thin sections, Monopora was named by Lebedev in 1924 based on internal features seen in a corroded specimen. The original illustration of M. donaica (Lebedev 1924, Pl. 1, fig. 2) was of a fenestellid fragment that in part showed clearly a single row of triangular to trapezoidal zooecial bases, while in other parts of the drawing what is intended to be portrayed is unclear. According to Nekhoroshev (1932, p. 295), "The third type of [basal fenestellid] section of zooecia appears three cornered. Peculiarities of construction of zooecia of this type as seen in a half destroyed state were mistakenly used by a single author (Lebedev) in the description of a new genus, Monopora, which is constructed the same as in Fenestella but inaccurately described as branches with only a single row of zooecia."

The generic name Monopora Lebedev, 1924 has been abandoned. In part, that is because Lebedev's original specimen has apparently been lost, and it is impossible to get a sense of zoarial or zooecial details from his description and illustration. Nikiforova (1933b), in a study of bryozoans from the same region and stratigraphic level, reassigned Monopora donaica to Fenestella and named three subspecies, minor, media, and major. Termier and Termier (1971) designated F. donaica (Lebedev) minor Nikiforova as type species of their new genus Alternifenestella. Lebedev (1924) gave no scale for the single illustration of Monopora donaica, so it is not possible to make any decision about which, if any, of the three subspecies named by Nikiforova (1933b) may include Lebedevs specimen. F. donaica (Lebedev) minor Nikiforova legitimately stands as type species of Alternifenestella.

The next genus initially diagnosed in part on internal structure of the branches was Minilya Crockford, 1944. However, between 1924 and 1944 Soviet bryozoan specialists vigorously adopted the use of tangential thin sections to supplement external morphological characterization of fenestrate species wherever possible (e.g., Nekhoroshev 1926, 1928, 1932; Nikiforova 1933a, 1933b, 1933c, 1938). Shul'ga-Nesterenko (1941) recognized five and later (Shul'ga-Nesterenko 1951) 14 morphological groups within the genus Fenestella based largely on morphology visible in thin sections, but – although she thought they likely existed – she never named any of the groups as either genera or subgenera.

Proliferation of New Genera. There have been two periods of relatively rapid proliferation of generic names for biserial fenestrates (Figure 1), the 1880s and the late 20^th century, especially the 1970s. The increase in the 1880s was due largely to the recognition of several different types of superstructures (Figure 1, 1880s arrow). In contrast the increase in names during the 1970s (Figure 1, 1970s arrow) was due almost entirely to characterization of new genera based on structures visible in thin sections, especially autozooecial morphology and presence of heterozooecia.

The change from biserial fenestrate generic concepts based almost exclusively on external morphology of zoaria and branches to an emphasis on internal morphology seen in thin sections derives in large part from Shul'ga-Nesterenko's work published in 1941 and later. Termier and Termier (1971) used Shul'ga-Nesterenko's informal groups based on details visible in thin sections to split off five new genera from Fenestella, although two of the five are invalid because no type species was designated from among the several species included in the intended new genus. This paper stimulated Morozova to revise the generic concept of Fenestella and to formalize several more of Shul'ga-Nesterenko's informal groups, and she named 10 new fenestellid genera (Morozova 1974). Since the 1970s almost all newly proposed genera of biserial fenestrates have been based largely or entirely on internal morphology visible in thin sections.

Characters Used in Generic Diagnoses. Features that are most clearly seen or are visible only in thin sections of species of Fenestella sensu latu include characters such as: 1) presence or absence of heterozooecia, 2) alignment of median obverse nodes in a linear or a zigzag row; and features of the autozooecia such as 3) presence or absence of hemisepta and (rarely) complete diaphragms, 4) angles of intersection of the transverse wall with the reverse and axial walls, 5) degree of internal overlap of the two rows of autozooecia along the branch and whether that degree of overlap is relatively constant from obverse to reverse side or increases toward the reverse side, 6) overall chamber size, and 7) the ratio of chamber length:width:height. Except presence or absence of heterozooecia and of hemisepta and diaphragms within autozooecia, these characters are continuous rather than discrete. Most characters visible on the skeletal surface are also continuous, such as 1) degree of branch sinuosity, 2) distance between dissepiments, 3) shape of fenestrules, 4) robustness of branches and 5) dissepiments, 6) presence of an axial keel, 7) spacing and 8) diameter of any nodes on the keel, 9) size and 10) position of autozooecial apertures and 11) their proximity to the median plane (i.e., keel, if present).

The vigorous subdivision of Fenestella s.l. into new genera, begun in the early 1970s, was not received well by V.P. Nekhoroshev, who along with his wife (A.I. Nikiforova) during the 1920s through 1940s had established inclusion of thin section observation as a matter of course for understanding characteristics of fenestrate species. He laid out his objections (Nekhoroshev 1979) in unambiguous terms in a paper titled "O neratsionalnosti razdeleniya roda Fenestella" ("On the non-rational subdivision of the genus Fenestella"). He considered the newly named genera to be junior synonyms of Fenestella and to be based on arbitrary and subjective boundaries within a single genus.

Indeed the majority of taxonomic characters within Fenestella s.l. are continuous, which means that some decisions about assignment of species to a genus split off from Fenestella s.l. are difficult to make. Characters and character combinations that make for difficult assignments include angle of intersection of the transverse interzooecial wall with the axial wall that divides the two rows of zooecia and with the reverse wall, as well as the degree of overlap of the chambers of the two rows of autozooecia along a branch. The overlap of autozooecial rows ranges from nonexistent (linear axial walls) to complete overlap basally, which results in triangular cross sections of chambers and no distinction between transverse and axial walls between autozooecia. In some cases (e.g., Ernst and Winkler-Prins 2008) a high proportion of fenestellid species in a study end up being questionably assigned to one or another of the fenestellid genera.