Small trilobites (usually only a few mm long) with cephalon and pygidium strongly similar in outline and size (isopygous); enrollment typical.
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 composed by segments 2 (Agnostina) or 3 (some Eodiscina), axis typically broad, short fulcrate pleurae. Strongly isopygous; pygidial margin typically closely matching cephalic margin.
From: http://www.trilobites.info/ordagnostida.htm

Ammagnostus laiwuensis

Ord. : AGNOSTIDA
Fam : AMMAGNOSTIDAE
Gen.: Ammagnostus ÖPIK, 1967

Order Agnostida is relatively uncommon in the Weeks Frm. and very rarely found.

Ammagnostus is characterized by glabella with ogival to subpentagonal anterior lobe. Pygidium with two opposite spine

Large, heterogenous order with primitive origins and problematic classification, with specialized offshoots that are hard to frame within a general diagnosis.

Cephalon typically with opisthoparian facial sutures, with gently forward-tapering simple glabella bearing a broad, rounded front, usually with 3 pairs of rather narrow parallel glabellar furrows; natant hypostome. Thorax typically large with 8+ thoracic segments. Pygidium quite variable, but typically with a small pygidium bearing a border (Cambrian) or a larger pygidium with or without border (post-Cambrian).
From: http://www.trilobites.info/ordptychopariida.htm

Cedaria minor, Walcott 1924b

Ord.: PTYCHOPARIIDA
Fam.: CEDARIIDAE
Gen.: Cedaria, WALCOTT, 1916
 

Cedaria is the mostest common trilobite found in the Weeks Frm. Preglabellar field well developed, librigenae with medium length genal spines. Thorax with 7 segments. Pygidium semicircular with 5 or 6 axial rings and shallow border furrows
 

Coosella sp. type 1 (up)
Coosella sp. type 2 (bottom)

Ord.: PTYCHOPARIIDA
Fam.: CREPICEPHALIDAE
Gen.: Coosella LOCHMAN, 1936

Rare trilobite, in the Weeks Frm are presents two different types of Coosella: type 1 and type 2.
Type 2 can be recognized by straighter thoracic profile and larger pygidium. Librigenae with short genal spines, thorax with 11 or 12 segments and deep furrows in the center of pleurae.
Surface finely granulose an typical pygidium with posterior median imbend.

Chiasmacantha cf. ceratops

Ord.: -
Fam.: -
Gen.: -

This is one of the rarest trilobites ever found in the Weeks formation.
Only know from a few number of specimens.
Characterized by large occipital spines and axial spines that “criss-cross” the axial ring.

Genevievella granulatus (Walcott, 1916)

Ord.: PTYCHOPARIIDA
Fam.: LLANOASPIDIDAE
Gen.: Genevievella, LOCHMAN, 1936

Rare trilobite with a large characteristic spine starting from the 7th axial ring.

Formerly "Democephalus" granulatus named by M. Beebe (1990) in this unbublished thesis. Distinct ridges eyes, long spines in the librigenae. The thorax present granulose surface; the pygidium is semicircular.

Densonella semele (Walcott, 1916) = Menomonia semele (Walcott, 1916)

Ord.: PTYCHOPARIIDA
Fam.: MENOMONIIDAE
Gen.: Densonella, SHAW, 1952

This trilobite is related to Dresbachia (see following specimen in this page) and Nephalicephalus. These species are elongates, with little pygidium (micropygous), eyes stalked, big number of thoracic segments (about 23 for Densonella genus) and surface granulose.

Dresbachia amata Walcott, 1916

Ord.: PTYCHOPARIIDA
Fam.: MENOMONIIDAE
Gen.: Dresbachia, WALCOTT, 1916a

This trilobite is related to Menomonia (see precedent images in this page) and Nephalicephalus. These species are elongates, with little pygidium (micropygous), and big number of thoracic segments(about 32 for Dresbachia genus).
 

Gerospina schachti (Shanan Peters dissertation, 2003, unpubl.)

Ord.: PTYCHOPARIIDA
Fam.: CEDARIIDAE
Gen.: Gerospina PETERS, 2003

G. sachti was discovered 10 years ago by Robert Schacht, a well known american collector of fossils. This trilobite was known only from fragments and assigned to genus Cedaria by M. Beebe (in the image a Cedaria without free cheeks overlap partially Gerospina). Only recent findings of complete specimens allow to put it in the genus Gerospina (Shanan Peters dissertation, 2003, unpublished).

Lonchocephalus plena

Ord.: PTYCHOPARIIDA
Fam.: LONCHOCEPHALIDAE
Gen.: Lonchocephal:us OWEN, 1852

Another rare trilobite not well known from the "grand publique"; this uncommon genus is small with only 7 to 10 thoracic segments.
The cephalon is characterized by the glabella deeply furrowed and with librigenal spines. Spines are present also in correspondence of the occipital ring and axial rings (center and posterior). In this specimen are well visible the three axial spines. Only few specimens are found and only in determinate layers of the Weeks Fm.

Maryvillia arion (WALCOTT, 1916)

Ord.: PTYCHOPARIIDA
Fam.: KINGSTONIIDAE
Gen.: Maryvillia WALCOTT, 1916b

This genus is extremely rare and it is relatively unknown. Rare are the complete specimens and only disarticulated parts are studied. This bluish wonderful specimen is one of the rarest complete that you can see on Internet.

More information on this genus is available in the paper F.Rasetti (1956) - Revision of the Trilobite Genus Maryvillia Walcott. Journal of Paleontology, Vol. 30, No. 5 (Sep., 1956), pp. 1266-1269, or at page O293 of the Treatise of Invertebrate Paleontology (reprint 1977), Part O.

Meteoraspis sp.

Ord.: PTYCHOPARIIDA
Fam.: TRICREPICEPHALIDAE
Gen.: Meteoraspis RESSER, 1935

Weeks Trilobites from the Carlo Kier collection II  >

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