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|ORDER AGNOSTIDA |
Last 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.
|Suborder Agnostina |
Cephalon: no facial sutures or eyes; glabella divided by transglabellar suture into anteroglabella and posteroglabella; cephalothoracic aperture present.
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.
|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.
| 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 of Olenoides, 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, click here.
Cotton, T.J. & R.A. Fortey. 2005. Comparative morphology and relationships of the Agnostida. In: Koenemann, S. & Jenner, R. (eds.). Crustacean Issues 16, Crustacea and Arthropod Relationships (CRC Press: Boca Raton).
Fortey, R. A. 2001. Trilobite systematics: the last 75 years. J. of Paleontology. 75(6):1141-51.
Fortey, R. A. & J. Theron. 1994. A new Ordovician arthropod Soomaspis and the agnostid problem. Palaeontology 37:841-61.
Jell, P. A. 2003. Phylogeny of Early Cambrian trilobites. Special Papers in Palaeontology 70:45-57.
Müller, K. J. & D. Walossek. 1987. Morphology, ontogeny and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden. Fossils and Strata 19:1-124.
Ramskold, L. & G. D. Edgecombe. 1991. Trilobite monophyly revisited. Historical Biology 4:267-83.
Shergold, L. H. 1991. Protaspid and early meraspid growth stages of the eodiscid trilobite Pagetia ocellata Jell, and their implications for classification. Alcheringa 15:65-86.
Notes on the planktonic life habits of Agnostida:
It has been suggested that Agnostida, via their small size, widespread distribution, and large numbers, might have been planktonic. However, there are a number of logical inconsistencies with this contention (via pers. comm. Brian Chatterton 2003):
1) Most Agnostida are blind, while most modern pelagic arthropods are sighted.
2) They are often found complete, articulated, right way up among complete articulated, right way up ptychopariid and other trilobites that everyone would accept as benthic.
3) Agnostid numbers often vary radically in different beds when collecting through a section (Richard Fortey, in his work on Spitsbergen, argued that pelagic forms should occur in small numbers throughout the section rather than in great numbers at some levels and few or none at others).
5) The widespread occurrence of species of agnostids does not require they be pelagic; deep cold water benthic species usually have much more widespread dispersal patterns than shallow shelf species (there have been a number of papers pointing this out for a wide range of organisms). Agnostids are typically associated with deep, cold water deposits.
6) The morphology of agnostids (low, flat bottomed, wide) is much more typical of benthic than pelagic forms. The latter are often as high as wide, and with obvious adaptations for powerful swimming appendages.
7) While agnostids are small, they are not so small that they would have been operating in a very low Reynolds number environment (hydrodynamically speaking). Therefore, to stay up in the water column, they would either have to work hard (to counteract the weight of their shell), or have some buoyancy mechanism to achieve neutral buoyancy. There is no evidence for this. Agnostid trilobites are not particularly thin shelled and some could even be argued to be moderately thick shelled.
8) Agnostid appendages (only known from Orsten deposits) may appear slightly unusual when compared with those of adult benthic trilobites. However, the only known appendages are for very small agnostids (meraspides). We do not know what the appendages of other trilobites at that stage would have looked like.
9) Most enrolled agnostids seem to be juveniles. Adult forms are almost all outstretched, and preserved dorsal side uppermost (if they are not disarticulated). This suggests that the mature stages at least were crawling around on the sea floor, dorsal side uppermost (not partly enrolled in the water column as suggested by some).
In summary, at the very least, agnostids spend some of their adult lives on the sea floor. They did not just come down to the sea floor to moult (otherwise we would only find moults in reasonable articulated form, the correct way up). Why did they not spend all of their adult lives on or close to the sea floor? The pelagic argument does not explain all of the data. Arguing that most or all agnostids were benthic, widespread, deep, cold water forms explains more of the available facts.
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