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Agnostids were very unusual trilobites, and it is difficult to tell their heads from their tails.One of the ways to tell is that the glabella (head region) usually consists of two main lobes,while the pygidial axis (tail region) usually consists of three lobes,one smooth lobe,or in the case of the Eodiscina,many segments.On either side of the posteroglabella (bottom center head region)are two small rounded bulges,called basal lobes.Another way to tell the difference is that if there are any spines on the pygidium (tail) they will be pointed downward.Most Agnostids were between 1 and 10mm long.Some scientists think they may have been planktonic or pelagic,which means they may have floated thru the water instead of swimming or crawling along the bottom,but there are some theories now which may disprove this belief.There are two suborders of Agnostids, the Agnostina, and the Eodiscina. The Agnostina did not have eyes, and had only 2 body segments between the cephalon(head) and the pygidium(tail), while the Eodiscinas had 2 or 3 segments,and some of them had eyes.And since the structure of the Agnostids' appendages shared many features with that of primitive crustaceans,many scientists believe that the Agnostid is not a true trilobite.There is also a belief that since Eodiscina had eyes and sutures that it shouldn't be classified as a suborder of Agnostida,but as a suborder of Ptychoparida belonging to the family Ellipsocephaloidea.The most current theory(Cotton & Fortey,2005 Comparative Morphology and Relationships of the Agnostida) ties the Agnostida firmly to the Eodiscine family Weymouthiidae,and also presents large amounts of compelling evidence supporting the Agnostids' rightful place as a true trilobite.Agnostids appeared during the end of the
lower Cambrian Period, and were already extinct by the end of the Ordovician Period.
Now, can you tell which one of these Agnostids
is heads up and which one is heads down?
 
The photo below of a Peronopsis interstrictus
(next to the green arrow)
is the actual size of the average Agnostid? (approx.10mm.)
 
STUDY QUESTIONS:
(See how many you can answer)
Answers are at the bottom of this page
DON'T PEEK!
1.During what Period did the Agnostids appear?
2.What are the two suborders of Agnostids?
3.How many eyes did the Agnostina have?
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