Phylogeny, systematics, and biostratigraphy of the Ordovician bryozoan genus Peronopora

Abstract

Specimens of Peronopora Nicholson, 1881, are abundant in Upper Ordovician rocks of the North American Midcontinent. Based on the positions of units in the Composite Conodont Standard Section, we have sampled 211 specimens over a stratigraphic interval of 9.1 million years. The average duration of sample spacing is 61,664 years but is commonly as small as 32,800 yr.Thirty-four morphometric characters were measured in each specimen and were converted into multistate characters; character-state breaks were established based upon each character's ability to discriminate between phenetic groupings. Each character was subsequently weighted based on the number of derived states, degree of independence from other attributes, and estimated heritability.Cladistic analysis of these data indicate that there are eight species in Peronopora each consisting of an optimally defined crown group and a basal stem group (or paraclade). Character states shared by stem and crown groups define species but, within species, stem and crown groups also differ in some character states. The species are, in ascending order from the base of the tree, Peronopora decipiens (Rominger, 1886), P. compressa (Ulrich, 1979), P. pauca Utgaard and Perry, 1964, P. milleri Nickles, 1905, P. horowitzi new species, P. vera Ulrich, 1888, P. sparsa Brown and Daly, 1985, and finally P. dubia (Cumings and Galloway, 1913). Diagnostic keys permit the unique assignment of each specimen to a species and the separation of members of stem groups from those of crown groups. Thirty-one characters are required to discriminate between all 211 specimens. This contrasts with previous studies of Peronopora where eight or fewer characteristics were used. Of the ten characters most useful in discrimination, only three had been used in the conventional species literature. This accounts, largely, for only 29.8 percent (51 of 171) of previously identified specimens being classified as members of the same species in this analysis. Discriminant function analysis of original measurements, using species identity as the grouping criterion, produces statistically significant separation of species.It appears that stratigraphic position had an explicit and undue effect on previous concepts of species many of which could not be recognized independently of stratigraphic position. All species of Peronopora appear, or are inferred to have appeared, within the Lexington Limestone between the base of the Grier Member and the top of the Millersburg Member. The cladogram indicates that species evolved in a sequential order, but their first appearance datums have been stratigraphically punctuated. Three species have ranges terminating in the Early to Middle Maysvillian, one in the Middle Richmondian, and four in the Late Richmondian. The latter four (or five) of these species died out in the extinction associated with the unconformity at the top of the Richmondian.

Cladistic analysis of the Paleozoic bryozoan families Monticuliporidae and Mesotrypidae

“…Additionally, hooked acanthostyles do not occur consistently in any of the other taxa, bifoliate species of Peronopora share a unique pattern of astogeny (colony development), and the genus has a restricted geographic distribution. Pachut and Anstey (2002) have supported Hickey's restriction of bifoliate species to Peronopora. Marintsch (1998) re-described several of the genera evaluated in this study (Peronopora, Prasopora, Mesotrypa, Homotrypa and Monticulipora) and established a new genus, Acantholaminatus, all of which were placed in the Monticuliporidae.…”

“…Historically, studies have considered most genera as members of the Monticuliporidae (Ulrich 1890, Astrova 1978, Anstey 1987, Pachut et al 1994, Marintsch 1998, Pachut and Anstey 2002, leaving the Mesotrypidae with the two genera (Diazipora and Mesotrypa) that Astrova originally included in her classification. However, one current taxonomic treatment (Bock 2010) placed most traditional monticuliporids in the Mesotrypidae (Table 1).…”

“…The concept of crown and stem groups was applied to analyses involving only fossilized taxa at the species level by Pachut and Anstey (2002).…”

The limited value of traditional morphometric features in stromatoporoid taxonomy

“…The estimation of character heritabilities was performed in previous studies of extinct species of bryozoans (Pachut , , ; Key ; Pachut and Anstey ; Hageman and Sawyer ). Total variance within populations of colonies was partitioned into inter‐colony (reflecting the additive genetic variance) and intra‐colony components.…”

“…The analysis of the variability of components has been undertaken by several researchers (Pachut , , ; Cheetham et al . , ; Pachut and Anstey ; Hageman and Sawyer ). Such studies are not generally possible for noncolonial organisms, although a relationship between the symmetry and genetic variability of solitary fossil taxa can be found (Palmer ), allowing us to estimate levels of genetic variability by calculating levels of fluctuating asymmetry.…”