ERA GEOLÓGICA ORDOVICICO

ORDOVICICO - ORDEN AGNOSTIDA .-

Tsunyidiscus     click on drawing  for pictorial guide	ORDER AGNOSTIDALast revised 07 December 2006 by S. M. Gon III Introduction: Small trilobites (usually only a few mm long) with cephalon and pygidium strongly similar in outline and size (isopygous); enrollment typical.  Cephalon: cephalic shield with deeply parabolic outline, maximum width usually anterior of genal angle, sutures proparian or lacking; border convex; glabella fusiform, widest at base (except in Condylopygidae), glabellar segmentation highly variable, sometimes complex, but in some species entirely effaced; most species eyeless; hypostome natant, specialized with ribbon-like wings; rostral plate lacking (or uncalcified).   Thorax: segments 2 (Agnostina) or 3 (some Eodiscina), axis typically broad, short fulcrate pleurae.   Pygidium: strongly isopygous; pygidial margin typically closely matching cephalic margin.   Occurrence: L. Cambrian to U. Ordovician (Ashgill).   Suborders: Agnostina and Eodiscina.
Agnostoidea  Agnostus     . . . . Condylopygoidea  Condylopyge	Suborder Agnostina Cephalon: no facial sutures or eyes; glabella divided by transglabellar suture into anteroglabella and posteroglabella; cephalothoracic aperture present.  Thorax: 2 thoracic segments bearing distinctive articulating structures, but no articulating half-ring on anterior thoracic segment.   Pygidium: axis usually wide, inflated, 3 or fewer segments, one of which usually carries a tubercle; posterior axial rings effaced (posteroaxis); pygidial margin often bearing posterolateral spines. Cuticle thin.   Superfamilies: Agnostoidea and Condylopygoidea (each described below). Superfamily Agnostoidea  Cephalon: with basal glabella lobes anteriorly directed, no clear occipital structure present, anterior glabellar lobe typically subequal in width to posteroglabella, or narrowing forward (not laterally expanded).  Thorax: as in typical Agnostina. Limbs of larval Agnostus pisiformis known; limbs unlike typical holaspid trilobite structure (see classification notes below).  Pygidium: variable within bounds of typical Agnostina. If not effaced, then often with two anterior segments (tubercle often on 2nd segment), and effaced posteroaxis. Families: Agnostidae, Ammagnostidae, Clavagnostidae, Diplagnostidae, Doryagnostidae, Glyptagnostidae, Metagnostidae, Peronopsidae, Ptychagnostidae.  Genera: Agnostidae: Acmarhachis (=Cyclagnostus; =Oxyagnostus; =Wanagnostus), Agnostus (/Battus; =Acutatagnostus), Aistagnostus, Anglagnostus, Biciragnostus, Connagnostus, Distagnostus, Eolotagnostus, Gymnagnostus, Homagnostus, Idolagnostus, Innitagnostus, Ivshinagnostus, Kymagnostus, Lotagnostus, Micragnostus, Obelagnostus, Oncagnostus (=Eurudagnostus), Phalacroma (=Platagnostus), Phalagnostus (=Phalacromina), Quadrahomagnostus, Raragnostus, Semagnostus, Strictagnostus, Trilobagnostus (=Rudagnostus).  Ammagnostidae: Ammagnostus (=Lispagnostus; =Agnostoglossa; =Tentagnostus), Hadragnostus (=Formosagnostus; =Kunshanagnostus), Kormagnostus (=Kormagnostella; =Litagnostoides), Proagnostus (=Agnostascus; =Paragnostascus).  Clavagnostidae: Aspidagnostus (=Biragnostus), Clavagnostus (=Tomorhachis; =Culipagnostus; =Stigmagnostus; =Acanthagnostus; =Leptagnostus; =Paraclavagnostus), Triadaspis, Utagnostus.  Diplagnostidae: Agnostotes, Baltagnostus (?Trilagnostus), Denagnostus, Diplagnostus (=Enetagnostus; =Tasagnostus), Dolichagnostus (=Enetagnostus; =Tasagnostus), Dolichagnostus, Iniospheniscus, Linguagnostus (=Cristagnostus), Machairagnostus, Nahannagnostus, Oedorhachis, Oidalagnostus (=Ovalagnostus), Pseudagnostus (=Rhaptagnostus; Sulcatagnostus; =Xestagnostus), Pseudogyptagnostus (=Glyptagnostotes), Pseudorhaptagnostus (=Neoagnostus; =Euplethagnostus; =Hyperagnostus; =Tarayagnostus; =Calagnostus), Trisulcagnostus (=Tririmagnostus).  Doryagnostidae: Doryagnostus (=Ceratagnostus; =Rhodotypiscus).  Glyptagnostidae: Agnostardis, Glyptagnostus (=Barrandagnostus).  Metagnostidae: Arthrorhachis (=Metagnostus; =Girvanagnostus), Chatkalagnostus (=Oculagnostus), Corrugatagnostus (=Segmentagnostus; =Granulatagnostus; =Cenagnostus), Diplorrhina (=Mesospheniscus; =Quadragnostus; =Pseudoperonopsis), Dividuagnostus (=Pezizopsis), Galbagnostus, Geragnostella, Geragnostus (=Geratrinodus; =Neptunagnostella), Granulagnostus, Homagnostoides, Novoagnostus, Trinodus.   Peronopsidae: Archaeagnostus, Cotalagnostus, Eoagnostus, Gratagnostus, Hypagnostus (=Spinagnostus; =Cyclopagnostus; =Breviagnostus; =Metahypagnostus), Lisogoragnostus (=Abagnostus; =Scanagnostus), Micagnostus, Peronopsella, Peronopsis (/Mesagnostus; =Euagnostus; =Acadagnostus; =Axagnostus; =Itagnostus), Sphaeragnostus, Svenax.   Ptychagnostidae: Allobodochus, Criotypus, Goniagnostus, Lejopyge (/Miagnostus), Myrmecomimus, Onymagnostus (=Agnostonymus), Pseudophalacroma, Ptychagnostus (=Triplagnostus; =Huarpagnostus; =Solenagnostus; =Pentagnostus; =Aristarius; =Aotagnostus; =Aciduscus; =Canotagnostus; =Zeteagnostus), Schismagnostus, Tomagnostella, Tomagnostus. Superfamily Condylopygoidea  Cephalon: with transversely oriented basal glabella lobes, separated by medial plate, together forming a clear occipital structure, anterior glabellar lobe laterally expanded around anterior end of posteroglabella, sometimes separated by a median sulcus.  Thorax: as in typical Agnostina.  Pygidium: axis with 3-segmented anteroaxis; broad, posteriorly rounded posteroaxis.  Family: Condylopygidae  Genera: Condylopyge (/Paragnostus; Fallagnostus), Miraculaspis, Pleuroctenium (=Dichagnostus). The arrangement of genera and families for suborder Agnostina is via Jell and Adrain 2003, superceding some of the classification of Agnostida in the 1997 Treatise. For listed genera, synonyms are shown in parentheses.
EodiscoideaEodiscidae Pagetia  Weymouthiidae Serrodiscus	Suborder Eodiscina Introduction: Eodiscina provide a bridge linking Agnostida to "typical" trilobites. Presence of eyes, sutures, and typical thoracic structure suggest that Eodiscina (especially Pagetiidae) are akin to Ptychopariida, but overall similarity to Agnostina (and placement in Agnostida) prevails in the 1997 systematic treatment of the current Treatise.  Cephalon: with proparian sutures, when present; some retain eyes; glabella usually simple, narrow and tapering.   Thorax: 2 or 3 thoracic segments of normal trilobite form, with articulating half-rings   Pygidium: axis can be long and narrow, extending close to margin, generally distinctly divided into more than 3 ringlike segments (to 11+); pleural region segmented in some species, sometimes pygidial segmentation effaced; border furrows often very deep; axis sometimes spinose.   Superfamily/Families: Eodiscoidea/Calodiscidae, Eodiscidae (including Pagetiidae), Hebediscidae, Tsunyidiscidae, Weymouthiidae, Yukoniidae.   Genera: Calodiscidae: Calodiscus (/Goniodiscus; =Brevidiscus), Chelediscus, Korobovia, Neocobboldia (/Cobboldia; =Margodiscus), Pseudocobboldia, Sinodiscus (=Tologoja).   Eodiscidae: Dawsonia (=Aculeodiscus; =Metadiscus), Eodiscus (=Spinodiscus; =Deltadiscus), Helepagetia, Kiskinella, Macannaia, Opsidiscus (/Aulacodiscus), Pagetia (=Eopagetia; =Mesopagetia), Pagetides (=Discomesites), Sinopagetia.  Hebediscidae: Delgadella (=Alemtejoia; =Delgadodiscus; =Delgadoia; =Pagetiellus; =Pentagonalia), Dicerodiscus, Hebediscus, Luvsanodiscus, Natalina (=Limbadiscus), Neopagetina (/Pagetina), Parapagetia (=Planodiscus), Tchernyshevioides.  Tsunydiscidae: Tsunyidiscus (=Mianxiandiscus; Liangshandiscus; =Emeidiscus; =Hupeidiscus; =Shizhudiscus; =Guizhoudiscus).   Weymouthiidae: Abakolia (=Costadiscus), Acidiscus, Acrimetopus, Analox, Bathydiscus, Bolboparia, Cephalopyge, Cobboldites, Leptochilodiscus (=Kerberodiscus), Litometopus, Mallagnostus (=Ladadiscus; ?=Jinghediscus), Meniscuchus, Ninadiscus, Oodiscus, Runcinodiscus, Semadiscus, Serrodiscus (=Paradiscus), Stingmadiscus, Tannudiscus, Weymouthia.  Yukoniidae: Alaskadiscus, Egyngolia (=Mongolodiscus), Ekwipagetia, Hebediscina (=Szechuanaspis; =Zhenbadiscus), Lenadiscus, Yukonia, Yukonides.
	CLASSIFICATION NOTES FOR AGNOSTIDA:  When the limbs of larval Agnostus pisiformis (see figure at right) were first documented (Muller and Walossek 1987), they proved very different from previously described trilobite limbs (e.g., those ofOlenoides, Triarthrus, Ceraurus, Phacops, etc.). This cast doubt about the presumed inclusion of the Suborder Agnostina within the Class Trilobita. However, no appendages from fully grown holaspid agnostines have ever been documented, leaving room for the contention that larval limbs may not resemble those of adults. The argument has also been made that agnostid trilobites may represent evolution via progenesis (maturity achieved while the body retains immature morphology), which suggests that even adult agnostines could retain the presumed larval form. No limbs of protaspides or meraspides of "conventional" trilobites has been described either, so the speculation that the limbs of Agnostus pisiformis might resemble the larval limbs of other trilobites remains untested.  By themselves, members of Agnostina can be treated in cladistic analyses so that they fall outside of the trilobite clade (Ramskold & Edgecombe 1991), however, with Eodiscina included, Agnostina+Eodiscina fall within the trilobite clade (Fortey & Theron 1994). Eodiscines provide characters that tend to link Agnostida with traditional trilobites (for example, they bear eyes, facial sutures, and conventional thoracic articulation). The ontogeny of eodiscines bears a number of differences from agnostines, leading Shergold (1991) to argue that they are not closely related. Limbs of Eodiscina have not been documented, and these might help clarify the relationship between Agnostina and Eodiscina. If their limbs are more like those of other trilobites, it would further distance the two Agnostida suborders. Alternately, if they prove similar to those of Agnostus, it would help cement the two suborders and bridge the Agnostina with typical trilobites.  Fortey (2001) included the Agnostina within the Trilobita in an unsettled manner. Jell (2003) argues for an origin of the Agnostida via the early eodiscoid Tsunyidiscus via progenesis from Bulaiaspis or other Bigotinidae (an early ellipsocephaloid family). This would tie the Eodiscina firmly to Trilobita, via early Ptychopariida, with the argument that the Agnostina are derived from Eodiscina. Cotton & Fortey (2005) conducted a phylogenetic analysis that strongly tie Agnostina to Eodiscina via the eodiscine family Weymouthiidae, concluding that Agnostina are indeed trilobites. For a brief summary of this paper