Trilobite Tip of the Month:

June 2001:

The Alokistocare / Amecephalus / Altiocculus problem:

  Amecephalus packi, Upper Glossopleura Zone, Chisholm Shale, Lincoln Co. Nevada
photo courtesy Andrew Milner

  Amecephalus cf. packi, Upper Glossopleura Zone, Chisholm Shale, Lincoln Co. Nevada
photo courtesy Andrew Milner

Amecephalus piochensis, Upper Glossopleura Zone, Chisholm Shale, Lincoln Co. Nevada
photo courtesy Andrew Milner

Altiocculus drumensis, Upper Wheeler Shale Formation, Millard Co., Utah
left photo courtesy Andrew Milner, right Kevin D. Brett
 

    The relationships of Cambrian ptychopariid trilobites are poorly known and difficult to determine based mainly on generalized and convergent morphologies of this group of trilobites. Alokistocaridae Resser 1939 is no exception to this. To date, over 85 species have been assigned to the genus Alokistocare Lorenz 1906, of which 56 are still valid (Sundberg, 1999). This was due, in part, to the poorly known morphology of the type species, A. subcoronatum (Hall and Whitfield, 1877). Howell (in Harrington et al., 1959) assigned 30 genera to Alokistocaridae Resser 1939, only a few of which have been reassigned to other families.
    For those collectors with North American species of Alokistocare, you may want to check the species below to see if they have been reassigned to different genera. Although the list is far from complete, for those interested in further information, please use the references listed at the end.

    A recent phylogenetic analysis by Sundberg (1999) using a matrix of 50 characters and 19 taxa resulted in the following conclusions:

1.    Alokistocaridae Resser 1939 is a monophyletic clade (and is not synonomous with Papyriaspididae Whitehouse 1939)
2.    Ehmaniellidae Sundberg 1994 is synonomous with Alokistocaridae
3.    Pseudoalokistocare Sundberg, 1994 is synonomous with Alokistocare
4.    Taxa previously assigned to the older "Alokistocare"/Amecephalus group (Plagiura to Glossopleura biochrons) form a separate clade from both Alokistocare and the Alokistocaridae. These older species are assigned to Amecephalus Walcott 1924

Alokistocare is restricted to the Marjuman Stage
Amecephalus is restricted to the Delamaran Stage (Eokochaspis nodosa biozone to Glossopleura Biozone)
Strotocephalus Resser, 1935 = Amecephalus

Genera included in the Alokistocaridae (by Sundberg, 1999) are: Alokistocare, Altiocculus, Ehmaniella, Elrathiella, Proehmaniella and probably Schopfaspis. Genera previously assigned to Ehmaniellidae, including Ehmania, Elrathia, Parehmania, Pseudomexicella, Trachycheilus, and Tympanuella may also belong to Alokistocaridae.

Amecephalus differs from Alokistocare in that it has a poorly developed occipital ring and furrow; poorly defined anterior border furrow; strap-like posterior area of the fixigena; and a pygidium with poorly defined anterior pleural bands that do not project from the border area.

Alokistocare includes the type species, A. subcoronatum (Hall and Whitfield, 1877), A. swasii, A. paranotatum, and others.

Amecephalus includes the type species, A. piochensis (Walcott, 1886), A. arrojosensis (Lochman, 1952), A. agnesensis (Walcott, 1912), A. cleora (Walcott, 1917), A. gordonensis (Resser, 1935), A. laticaudatum, A. idahoense, A. packi, and others.

Altiocculus includes A. concavus, A. drumensis, A. harrisi, A. americanus, and others.

    The systematics of this family is complicated and attempts to classify the trilobites into various subfamilies has not been widely adopted. Although this recent cladistic study resolves the relationships of some alokistocarid species, it does not tackle a revision of all species formerly assigned to Alokistocare.

References:

Sundberg, F. A. (1999)  Redescription of Alokistocare subcoronatum (Hall and Whitfield, 1977), the type species of Alokistocare, and the status of Alokistocaridae Resser, 1939b (Ptychopariida: Trilobita, Middle Cambrian). Journal of Paleontology, Vol. 73, no. 6, pp. 1126-1143.

Sundberg, F. A. and McCollum, L. B. (2000)  Ptychopariid trilobites of the Lower-Middle Cambrian boundary interval, Pioche Shale, southeastern Nevada. Journal of Paleontology, Vol. 74, no. 4, pp. 604-630.