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Systematic Descriptions of the Class Trilobita

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Systematic Descriptions of the Class Trilobita W SYSTEMATIC DESCRIPTIONS OF THE CLASS TRILOBITA H. B. WHITTINGTON, W. T. CHANG, W. T. DEAN, R. A. FORTEY, P. A. JELL, J. R. LAURIE, A. R. PALMER, L. N. REPINA, A. W. A. RUSHTON, and J. H. SHERGOLD [Diagnoses of ordinal and higher taxa by R. A. FORTEY and H. B. WHITTINGTON; authorship of other diagnoses indicated within.] Class TRILOBITA Walch, 1771 cephalic. Lower Cambrian (Atdabanian)— [nom. correct. rxTrilobiten WALCH, 1771, p. 120] [This name, as Upper Permian. "Trilobiten,* was published by WALCH (1771, p. 120) in his volume III of KNORR and WALCH (1768-1774). WALCH'S work was rejected for nomen- datorial purposes by the International Commission on Zoological Nomen- Order AGNOSTIDA Salter, 1864 clature (Opinion 529, 1958). However, the Commission's remit extends only to the family-group level (Article 1 b(4), International Code of Zoologi- [nom. correct. SHERGOLD, LAURIE, & SUN, 1990, p. 32, pro Agnostini cal Nomenclature, 1985). hence the name, emended to "Trilobita," remains SALTER, 1864a, p. 2] [*Miomera JAEKEL, 1909, p. 394; Agnostida available. The designation of the genus Dalmanites BARRANDE as type KOBAYASHI, 1935, p. 81; see discussion in ROBISON Be CAMPBELL, 1974, p. (HENNINGSMOEN & MOORE in MOORE, 1959, p. 172) is invalid.] 281] Extinct marine arthropods, body divided Diminutive, isopygous, 2 or 3 fulcrate by furrows longitudinally into median (axial) thoracic segments. Cephalic shield with and lateral (pleural) regions, transversely into deeply parabolic outline and maximum cephalon of fused segments, thorax of articu- width (tr.) usually anterior to genal angle; lated segments, and pygidium of few to border convex; glabella commonly fusiform, many fused segments. Length of mature in- widest at base (except in Condylopygidae). dividuals ranges from a little over 1 mm to Hypostome natant; rostral plate lacking or about 700 mm. uncalcified. Outline of pygidium closely Dorsal exoskeleton and periphery of ven- matching that of cephalon. Lower Cam- ter (doublure) calcified (but see comments brian-Upper Ordovician. on Naraoiidae); calcified plate (hypostome) anterior to mouth and below anterior axial Suborder AGNOSTINA region. Anterior wing of hypostome extend- Salter, 1864 ing dorsally to a fossula in the axial furrow. [nom. correct. SHERGOLD, LAURIE, & SUN, 1990, p. 32, pro Agnostini Facial suture characteristic, absent in SALTER, 1864a, p. 2] Olenellina, but may be secondarily lost or No eye or facial suture; 2 thoracic seg- modified. Rostral plate or median suture ments; cephalothoracic aperture present; present, or cheeks conjoined. Cephalon with pygidium generally with wide axis, margin one pair of eyes (which may be lost), each eye commonly bearing posterolateral spine. Cu- lobe connected to axial region by raised ticle thin. Calcified protaspis not known. ridge; eye lenses of crystalline calcite. Slightly Lower Cambrian—Upper Ordovician. oblique-transverse pleural furrows at poste- rior of cephalon, on each thoracic segment, INTRODUCTION TO THE and on pygidial segments. Growth of exo- SUBORDER AGNOSTINA skeleton proceeded by molting from calcified protaspis about 1 mm long to adult, and by JOHN H. SHERGOLD and JOHN R. LAURIE sequential release of segments into thorax. The historical background to the classifi- Protaspides of Olenellina and Agnostina not cation of this suborder has recently been dis- known. Limbs (where known) include one cussed in detail (SHERGOLD, LAURIE, & SUN, pair of antennae, followed postorally by se- 1990, p. 1-3). The classification presented ries of generally similar biramous limbs, of here is as developed in that paper and essen- which three (possibly four in one species) are tially follows ideas on relationships that have Olenellina—In troductio n 405 short (sag.) or absent (Fig. 30.1). Thorax glabella has parallel sides or tapers forward, nonfulcrate. Pygidium narrow (tr.), with few LA is short, and the ocular lobe is attached segments. Calcified protaspis unknown; ear- along the entire margin of LA (Fallotas- liest meraspis with segmented interocular pidinae, Daguinaspidinae). In later genera, area. Lower Cambrian. LA first becomes elongate, so that the ocular lobes connect only to its posterior part INTRODUCTION TO SUBORDER (Judomiidae, Nevadiidae), and then gener- OLENELLINA ally expands laterally, and the glabella as a whole expands anteriorly from the level of SI A. R. PALMER and L. N. REPINA (Olenellidae, Holmiidae). LA in these genera Olenellina are a morphologically varied is also commonly inflated. Accompanying and highly diverse group of generally this modification, the distal parts of L3 ex- micropygous trilobites that share a primary tend laterally and posterolaterally and en- absence of facial sutures, the presence of a croach on L2, often isolating the S2 furrows well-developed ocular lobe at all develop- (Fig. 255), and L3 takes on a broad M-shape. mental stages, and an ontogeny in which the This phylogenetic trend underlies the pro- first mineralized stages are already early posed classification of the superfamilies. meraspids (PALMER, 1957). They are re- Within the Fallotaspidoidea, all Fallotas- stricted to and characteristic of rocks of later pididae, the earliest family of the Olenellina, Early Cambrian age and constitute a major have the ocular lobe attached along the en- suborder within the order Redlichiida tire margin of LA and an unmodified L3, (MOORE, 1959). More than 50 genera or and the anterior end of LA does not project subgenera and their associated higher taxa forward of a line tangent to the anterolateral are recognizable (PALMER & REPINA, 1993). margin of the ocular lobe. In the remaining, These taxa form the principal basis for bio- generally younger families of the Fallotas- stratigraphic subdivisions (Fig. 254) of the pidoidea (Archaeaspididae, Judomiidae, later Lower Cambrian rocks of Laurentia Neltneriidae, Nevadiidae), the anterior end (North America exclusive of the eastern sea- of LA projects forward of the junction with board from Newfoundland to Florida and the anterior margin of the ocular lobe, but including Spitsbergen and northwestern L3 remains unmodified. All Olenelloidea, Scotland) and are major indices for the later which includes the youngest Olenellina, Lower Cambrian biostratigraphy of Baltica have the ocular lobe attached to the posterior (northern Europe exclusive of the British part of LA and have a modified L3. Isles), Avalonia (England, Wales, eastern Within the superfamilies of the Olenel- Newfoundland, Nova Scotia, eastern New lina, morphological changes of taxonomic England, USA), Siberia, and the Moroccan value at the family level follow different pat- sector of Gondwana (including Spain). terns. The principal character distinguishing Ancestry of the Olenellina can only be the two families in the Olenelloidea is the speculative. They first appear as fully devel- relationship between the extraocular area and oped and morphologically diverse trilobites the interocular area. The Holmiidae all have in the early, but not earliest, part of the shelly a narrow extraocular area that is less than fossil record and are the oldest trilobites twice the width of the interocular area. With known. The principal phylogenetic trend minor exceptions (Olehelloides and some within the Olenellina involves the relation- undescribed Laurentian forms), the Olenel- ship between the ocular lobe and the frontal .lidae have a wide extraocular area that is (anterior) lobe (LA) of the glabella (REPINA, more than twice the width of the interocular 1990a). In all of the earliest Olenellina, area. In the Fallotaspidoidea, the relationship which include forms from Siberia, Laurentia, between the ocular lobe and LA, the shape of and the Moroccan sector of Gondwana, the the glabella (parallel sided versus tapered), Olenellina—Ollenelloidea 409 Mummaspis FRITZ, 1992, p. 17 [* Wanneria occidens opposite SI; genal spine at posterolateral corner or WALCOTT, 1913b, p. 314; OD; holotypc slightly advanced, strongly inflated. Opisthothorax (WALCOTT, 1913b, pi. 53, fig. 2), 60080, USNM, of at least 10 segments. Lower Cambrian: USA Washington, D.C.]. Parts of external surface reticu- (California, Nevada), upper Olenellus Zone. late. Posterior margin of cephalon nearly straight; FIG. 257,2. *P. iddingsi (WALCOTT), California; intergenal swelling slightly distal to midlength of nearly complete individual, LACMIP 1162/, X2 posterior margin. Preglabellar field absent or length (Palmer & Repina, 1993, fig. 4.2). 2_ (sag.) less than that of border; S3 deep, continuous across glabella; occipital spine may be present. Ocu- Subfamily BRISTOLIINAE lar furrow deep; outer band of ocular lobe narrower than inner band. Third thoracic segment generally Palmer & Repina, 1993 only weakly macropleural. Lower Cambrian: [Bristoliinae PALMER & REPINA, 1993, p. 23] [-Olenellinae POULSEN in Canada (southern Rocky Mountains), lower MOORE, 1959, p. 162, partim; Olenellidae BERCSTROM, 1973b, p. 312, Zone. FIG. 256,la,b. partim; AHLBERC, BERCSTROM, & JOHANSSON, 1986, p. 40, partim; Olenellus *M. occidens Fremontiinae REPINA, 1979, p. (WALCOTT); a, complete individual, topotype, 22, partim] USNM 443745, X4 (Fritz, 1992, pi. 9, fig. 2); b, Glabella usually strongly constricted at S1 cephalon and partial thorax, topotype, USNM or L2; width (tr.) of anterior part of LI usu- 433750, XI.7 (Fritz, 1992, pi. 10, fig. 2). ally distinctly narrower than occipital ring; Subfamily BICERATOPSINAE glabellar furrows generally well developed. Pack & Gayle, 1971 Preglabellar field shorter (sag.) than
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