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Encrinurus" Variolaris Plexus (Trilobita)."

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Encrinurus AMERICAN MUSEUM NornItates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2968, 22 pp., 9 figs. April 25, 1990 Mackenziurus, a New Genus of the Silurian "Encrinurus" variolaris Plexus (Trilobita) GREGORY D. EDGECOMBE' AND BRIAN D. E. CHATTERTON2 ABSTRACT Mackenziurus reimeri n. gen. n. sp., from Wen- common ancestry with Balizoma Holloway is a lock strata in the Mackenzie Mountains, North- competing hypothesis. Paraphyletic "Nucleurus" west Territories, Canada, is designated type species Ramsk6ld groups the sister taxa of Balizoma, ofa new genus ofthe "Encrinurus" variolaris plex- Mackenziurus + Fragiscutum, and Frammia Hol- us. Mackenziurus n. gen. also includes Wenlock tedahl. A silicified growth series for Mackenziurus species from Wisconsin-Illinois and Arkansas. A reimeri provides new data on ontogenetic trans- sister-group relationship with Fragiscutum Whit- formations for the variolaris plexus. tington and Campbell is suggested; more recent INTRODUCTION The systematics ofSilurian encrinurine tri- whereas E. (Nucleurus) Ramsk6ld, 1986a, lobites were set in an explicit genealogical represents the plesiomorphic Llandovery core framework by Strusz (1980) who distin- of the variolaris plexus. The latter group was guished three "phylogenetic plexi" within envisioned as ancestral to restricted Wen- Encrinurus Emmrich, 1844. Ramskold's lock?-Ludlow offshoots (Fragiscutum Whit- (1986a) subgeneric revision of Encrinurus tington and Campbell, 1967; Frammia Hol- formalized these relationships: E. (Encrinu- tedahl, 1914) as well as to geographically rus) and E. (Pacificurus) Ramsk6ld, 1986b, widespread and morphologically conserva- comprise the Euramerican punctatus and tive Wenlock-Ludlow Balizoma Holloway, Australasian mitchelli plexi, respectively, 1980. ' Graduate student, Department of Invertebrates, American Museum of Natural History; and Department of Geological Sciences, Columbia University. 2Professor, Department of Geology, University of Alberta, Edmonton, Alberta, Canada T6G 2E3. Copyright © American Museum of Natural History 1990 ISSN 0003-0082 / Price. $2.75 2 AMERICAN MUSEUM NOVITATES NO. 2968 a b c d e f k I m a b c d e f h i i k I m 3 j k I m A 0 0 I ICI Fig. 1. Cladograms expressing component information retrieved from encrinurine classification (com- ponents analysis as outlined by Nelson and Platnick, 1981). A. Derivative cladogram from unsequenced generic list classification (Strusz, 1980: 8; cf. Hamada, 1961; Evitt and Tripp, 1977) predicts no com- ponent information additional to general component 0 (Encrinurinae). B. Cladogram expressing hy- pothesis of phylogenetic relationship for the same terminal taxa (from Strusz, 1980: fig. 9) is more informative, predicting three components. Unresolved polytomies within components 0, 1, and 3 result partly from paraphyletic ancestral taxa (0-Encrinuroides; 1 -Coronocephalus; 3-Encrinurus; see C for example). C. Hypothesis of relationships between terms of component 3 (from Strusz, 1980: fig. 9); Encrinurus ("i") is paraphyletic. Terminal taxa abbreviated as follows: a, Encrinuroides; b, Physema- taspis; c, Cromus; d, Coronocephalus; e, Kailia; f, Senticucullus; g, Erratencrinurus; h, Celtencrinurus; i, Encrinurus; j, Paraencrinurus; k, Batocara; 1, Fragiscutum; m, Frammia. The present survey offers an assessment of Terminology applied to encrinurine mor- variolaris plexus relationships and taxonomy phology follows Evitt and Tripp (1977), with incorporating methods of phylogenetic sys- the exception that the conventional "axis" is tematics. We have thus attempted to recog- used rather than "rachis." Description of en- nize monophyletic taxa (sensu Hennig, 1966) crinurine early ontogeny is as outlined by based on synapomorphies, and to integrate Edgecombe et al. (1988). The preglabellar lat- these components into the hierarchical struc- eral lobe is termed PL following Howells ture of classification. The information con- (1982), and description of glabellar furrows/ tent (Mickevich and Platnick, 1989) of cur- lobes is as recommended for the revision of rent encrinurine classifications is adversely the Treatise on Invertebrate Paleontology, affected by paraphyletic ancestral taxa (see Part 0 (H. B. Whittington and S. R. A. Kelly, fig. 1). A distinctive cluster of North Amer- unpubl.). Ramskold's (1986a) notations for ican Wenlock species of the variolaris plexus fixigenal "circumocular tubercles" and ter- is herein recognized as a monophyletic tax- minal pygidial pleural ribs, as well as pygidial on and are described as the new genus Mack- "R/P ratio," are adopted. "Major row" and enziurus, with type species M. reimeri n. sp., "inter-row" glabellar tubercles are as intro- from the Wenlock of the Mackenzie Moun- duced by Edgecombe and Chatterton (1987). tains, Northwest Territories, Canada. Certain Orientations use the typical abbreviations for Wenlock species from Illinois, Wisconsin, and sagittal (sag.), exsagittal (exsag.), and trans- Arkansas previously referred to Encrinurus verse (tr.). and Balizoma are reassigned to Mackenziu- rus n. gen. Type and figured specimens of ACKNOWLEDGMENTS Mackenziurus reimeri are housed in the pa- leontological type collections of the Univer- This study was funded by operating grants sity of Alberta (UA). from the Natural Sciences and Engineering 1 990 EDGECOMBE AND CHATTERTON: MACKENZIURUS 3 Research Council of Canada, and Energy, and an adaxially narrowing cranidial anterior Mines and Resources (Canada) to Chatterton border. Although the usefulness of "a vario- and from the Boreal Institute for Northern laris group" has been questioned on phenetic Studies to Edgecombe. We thank Dr. Lars grounds (Temple and Tripp, 1979), mono- Ramsk6ld and Mr. R. P. Tripp for helpful phyly of the variolaris plexus is supported by comments on a preliminary version of this these unique characters; we hold that mono- paper, as well as formal reviews. Thorough phyly is both a necessary and sufficient cri- criticism from Dr. D. J. Holloway forced us terion for a taxon's utility. to reevaluate many of our arguments. Pho- Enlarged L2-L4 lateral glabellar and adax- tographs of a well-preserved Mackenziurus ial fixigenal tuberculation, and reduced genal cephalon from the Racine Dolomite at Wau- spines, are derived characters uniting this watosa, Wisconsin, were kindly provided by plexus with Pacificurus Ramsk6ld. It must be L. Ramsk6ld. These, as well as loan of spec- noted, however, that the occurrence of these imens in the Milwaukee Public Museum col- states in certain Llandovery species with sev- lections (arranged by Dr. R. Watkins) and eral diagnostic characters of the punctatus correspondence with Mr. K. C. Gass, aided plexus (e.g., E. squarrosus Howells, 1982) greatly in the generic diagnosis. cautions that they may be symplesiomor- phies, and thus specify a more inclusive group SYSTEMATICS OF THE (Encrinurus sensu Ramsk6ld, 1986a). (That VARIOLARIS PLEXUS hypothesis would demand reversals in de- rived parts of the punctatus plexus.) Predict- Strusz's (1980) Encrinurus variolaris plex- ing the component variolaris plexus + Pa- us included species referred by Ramskold cificurus at the exclusion of the punctatus (1986a) to Encrinurus (Nucleurus) and Bali- plexus, Encrinurus (cf. Strusz, 1980: 54), zoma Holloway. Fragiscutum and Frammia questions the grouping of Encrinurus, Paci- were perceived as "offshoots," i.e., descen- ficurus, and "Nucleurus" as a unique taxon dants excluded from the plexus. This dis- (i.e., as subgenera of Encrinurus). Although tinction between ancestral and descendant that grade group might reflect the equivocal taxa (Haeckelian ancestry of Rieppel, 1988) relationships of certain Llandovery species, is not endorsed here; the variolaris plexus is it predicts that "Nucleurus" shares indepen- expanded to include all descendants of a dent history with Pacijicurus and Encrinurus unique common ancestor which would itself rather than with other parts of the variolaris be part ofthe group. "Nucleurus, " Balizoma, plexus. This statement [as retrieved from the Fragiscutum, Mackenziurus n. gen., and classification; derivative cladogram (Nelson Frammia inclusively form a monophyletic and Platnick, 1981) in fig. 2a] is incongruent group for which the informal name "vario- with the phylogenetic hypothesis endorsed by laris plexus" is retained. Restricting this group Ramskold (1986a) and herein that "Nucleu- to an ancestral/primitive core (e.g., "Nucleu- rus" is most closely related to other genera rus," with or without Balizoma) but exclud- of the variolaris plexus (fundamental clado- ing Frammia and Fragiscutum (based on their gram in fig. 2b). Following Ramskold's uniquely derived characters) results in a para- (1986a) diagnoses, Encrinurus (the punctatus phyletic group. Schrank's (1972) synonymy plexus) and Pacificurus are treated as separate of Fragiscutum and Reed's (1928) Encrinu- generic taxa herein to eliminate the paraphy- rus variolaris group with Frammia would letic "Encrinurus" created by grouping them eliminate several generic names widely used with plesiomorphic parts of the variolaris by subsequent workers. It seems most infor- plexus (the "Nucleurus" grade). This problem mative to retain existing names for express- of paraphyly is only compounded when we ing relationships between parts of the more entertain excluded (autapomorphic) sister inclusive variolaris plexus. groups for the other "subgenera" ofEncrinu- Characteristic of the variolaris plexus are rus.
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