Asaphida

Taxobox
name = Asaphid trilobites

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image_width = 240px
image_caption = "Asaphus lepidurus"
regnum = Animalia
phylum = Arthropoda
classis = Trilobita
subclassis = Librostoma
ordo = Asaphida
subdivision_ranks = Superfamilies
subdivision =
Anomocaroidea
Asaphoidea
Dikelokephaloidea
Remopleuridoidea
Cyclopygoidea
Trinucleioidea

Asaphida is a large, morphologically diverse order of trilobite found in strata dated from the Middle Cambrian boundary to the Silurian. The order contains six superfamilies, but no suborders. Asaphids comprise some 20% of described fossil trilobites.

The Asaphida generally have cephalon (head) and pygidium (tail) parts similar in size, and most species have a prominent median ventral suture. Heads are often flat, and carapace furrows in the head area are often faint or not visible. Thoracic segments typically number 5 - 12, though some species have as few as two and some as many as 30. They also generally have a wide doublure, or rim, that surrounds the cephalon. This causes some specimens to be described as having a characteristic "snowplow" shaped cephalon. Eyes are typically large.

One Asaphida line, the superfamily Asaphoidea, shows a continuous evolution of eyestalks, from ones with mere eye stubs to "Asaphus kowalewskii", a trilobite popular with collectors that sported long, eye-tipped stalks. This line is found in the Middle Ordovician Asery Level deposits of the Wolchow River region near Saint Petersburg, Russia. During the Ordovician, what is now Eastern Europe was a shallow inland sea. This eye-stalk development is believed to be an adaptation to changes in turbidity during this time, with eye-stalked trilobites like "A. kowalewski" presumably arising in a time of increased turbidity. One thought is that this trilobite may have lain in wait for prey buried in the bottom sediment with only its periscope eyestalks protruding.

The major extinction event marking the end of the Ordovician Period reduced the diversity of all trilobite orders with most Asaphid families disappearing. The only surviving Asaphids were members of superfamily Trinucleioidea, and they too disappeared before the end of the Silurian Period.