Alokistocaridae n.sp., aff. Piochaspis sp.

 

This is a rare Middle Cambrian trilobite from Nevada presented here in case someone is making a study on this new species of Alokistocarid.

The Chisholm Shale, Nevada, is one of these mythic names for all trilobite collectors. It is somewhat like Beecher's Bed, Rome NY or Bundenbach, Germany... Any one of us has heard of this place :) The most commonly encountered bugs there are : Amecephalus packi, Zacanthoides typicalis, Piochaspis sellata, Glossopleura sp., etc.

For a better description of this trilobite I use the diagnosis of Piochaspis sellata made by Riccardo Levi-Setti (in "Trilobites, second edition". The University of Chicago Press. 1993) .

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Specimen Description :

  • Positive and negative part.
  • On matrix.
  • 95 % complete.
  • Very similar to Piochaspis sellata, but with genal spines slightly bent inward with about a 20° angle.
  • Total size : 11 mm.
  • Middle Cambrian.
  • Upper part of the Glossopleura Zone, Chisholm Shale.
  • Lincoln Co., Nevada, USA

 

 

 

 

 

Positive part
Negative part, with exoskeleton.

 

Diagnosis :

 

Order

 

 

 

 

 

Ptychopariida

 

 

 

 

SWINNERTON 1915

 

 

 

 

 A large, heterogenous order, with classification problems. Occurence : Cambrian to Upper Ordovician.
  • Cephalon : Generally bearing genal spines.
  • Typically opisthoparian facial sutures.
  • Gently forward-tapering simple glabella bearing a broad, rounded front.
  • Usually 3 pairs of rather narrow parallel glabellar furrows.
  • Natant hypostome (some forms conterminant).
  • Rostral plate shorter than the glabellar length.
  • Thorax : Typically large, with 8+ thoracic segments.
  • No fulcræ.
  • Pygidium : Quite variable, without noticeable structures.
  • Typically small, border with terrace lines (Cambrian forms). Larger pygidium, with or without border in the post-Cambrian forms.

Suborder

 

 

 

 

 

 

Ptychopariina

 

 

 

 

 

RICHTER 1933

 

 

 

 

Paraphyletic primitive Ptychopariida, a large and extremely plesiomorphic group.

  • Cephalon : Glabella usually tapering or barrel-like.
  • Usually 3 pairs of glabellar furrows.
  • Sutures typically opisthoparian (but some proparian, and blind forms marginal)
  • Anterior sutures usually convergent to slightly divergent, posterior sections moderately to highly divergent.
  • Eyes usually present, medial, and near glabella
  • Usually blade-like genal spines present.
  • Relatively narrow rostral plate.
  • Thorax : Generally long, relative to pygidium.
  • Pygidium : Micropygous to subisopygous.

Superfamily

 

Ptychoparioidea

MATTHEW 1887

 

Occurence : Cambrian to Ordovician. Forms with oval, elongate, exosqueleton.

  • Cephalon : Semi-circular.
  • Opisthoparian sutures.
  • Typically with well-defined border.
  • Glabella tapering forward, bearing 3 to 4 pairs of glabellar furrows.
  • Preglabellar field present, convex or flat.
  • Natant hypostome.
  • Short to medium sized genal spines.
  • Eye ridges usually present.
  • Exceptions include eyeless forms (Conocoryphidae), proparian forms (Norwoodiidae), rounded genal angles and gonatoparian sutures (Menomoniidae).
  • Thorax: Relatively broad.
  • Typically 12 to 17 segments.
  • Pygidium : Micropygous but exceptionally isopygous (e.g., some Asaphiscidae, Cooselidae).
  • Transverse, pleural field nearly flat, with distinct pleural grooves;
  • Few segments.
  • Devoid of border.

Family

Alokistocaridae

RESSER 1939

Occurence : Lower Cambrian to Upper Cambrian.

  • Cephalon : Preglabellar area wide.
  • Glabella somewhat taperring.
  • Facial sutures rather deeply concave opposite the glabella.
  • Eye ridges well developped.
  • Palpebral lobes small.
  • Genal spines of moderate size.
  • Pygidium : Micropygous.

 

To better understand the similarities to Piochaspis sellata, I report here the description made by R. LEVI-SETTI (1975) of this species :

 

Genus

Piochaspis

LEVI-SETTI 1975
  • Cephalon : Anterior border convex.
  • Anterior border furrow with median imbed.
  • Preglabellar field depressed mesially, elevated laterally.
  • Glabella tapering forward, rouded in front.
  • 3 pairs of shallow lateral furrows.
  • Axial furrows deep.
  • Facial sutures diverging in front of the eyes.
  • Eye ridges narrow, ~ 0.3 of glabellar width at midlength.
  • Palpebral areas convex, narrow anteriorly.
  • Posterior area of fixigena deeply furrowed.
  • Librigenae with pointed genal spines, flaring posteriorly at ~ 45°.
  • Thorax : 17 segments.
  • Pleurae grooved.
  • Pleural spines short, outward pointing.
  • Pygidium : Micropygous.
  • Devoid of border.
  • Prominent axis.
  • 1 axial ring.

Species

 

 

sellata

 

 

LEVI-SETTI 1975

 

 

 

Piochaspis sellata. Holotype. PE 54115. (R. LEVI-SETTI, 1993)
Same specimen, while immersed in xylene (better contrast due to different difractions) (R. LEVI-SETTI, 1993)

 

As the reader might see it, the frontal border seems to have a generally more distinct furrow, with a median imbed (very specific of Piochaspis sellata) that is much more less expressed. The anterior border outline is convex, maybe a bit more semi-circular. The glabella is similar to P. sellata. The facial sutures are diverging in front of the eyes in their proximal part, and converging in their frontal part. The eye ridges seem larger. Lastly, the genal spines bend slightly inward with a ~ 20° angle.

 

Study of the Glabella
Anterior border furrow and facial sutures.

 

Note on the following pictures the lack of pygidium and possibly the last thoracic segments. If not, this new Alokistocarid would only have 12 thoracic segments vs 17 for P. sellata.

 

B&W negative part.
B&W positive part.

 

Specimen found on the net :

This is the second specimen I know of, initially property of Pangeafossil :

Alokistocaridae n. sp., aff. Piochaspis.
Size : 11 mm.
Same specimen, B&W.

 

One can see here again the lack of pygidium and once again only 12 thoracic segments. The pleural thoracic tips are better shown here, but the genal spines are partial (the left one being lacking).

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