File:Agnostus pisiformis fossil trilobite (Alum Shale Formation, upper Middle Cambrian; Honsater, Vastergotland, Sweden) (15263805372).jpg

Agnostus pisiformis (Wahlenberg, 1818) fossil trilobite from the Cambrian of Sweden (line drawing from Wahlenberg, 1818).

Well-preserved trilobites occur in Sweden’s Alum Shale Formation (Middle to Upper Cambrian). The limestone beds in this unit are often trilobite packstones. Shown here is an early 19th century line drawing of the agnostoid trilobite Agnostus pisiformis from Hönsäter, Västergötland, Sweden.

Agnostoids differ from "ordinary" trilobites (= polymeroids) by having the following characeristics: two-segmented thorax; blind (no eyes); isopygous (the heads & tails are equal-sized). Agnostoids are usually quite small.

Many modern arthropod phylogenetic studies conclude that agnostoids should be excluded from the trilobites.

Classification: Animalia, Arthropoda, Trilobita, Agnostida, Agnostidae

Stratigraphy: Alum Shale Formation, near-uppermost Middle Cambrian

Agnostus pisiformis was first described & illustrated in the literature by Magnus von Bromell in 1729 and 1740, who described the species as "minimorum vermiculorum vaginipennium" ("small beetle-like worms").

Agnostus pisiformis was later described by the great Carl Linnaeus in his 1747 book Wästgöta Resa [Travels in Västergötland]: “The limestone was grayish and dug up at the Gösätter farm near the barn, where it occurs not far below the surface. Petrifactions were seen in all cracks on this rock in a way as though they looked like they had been sealed; but what kind is difficult to determine, because the shells were little larger than the seed of a palsternacke, and looking like a small snail shell; although all had the impression of a coleopteran insect. The upper side of this limestone was completely coarse and uneven and looked like a dried out bog mud surface, because it was gray, and covered by fine perpendicular lamellae that were oriented longitudinally and latitudinally, from the frost on the ground in the most severe winter. All this rock produced a stink, and was a Lapis suillus, or stinkstone. Often the stinkstone occurred as round balls in the common limestone; but in the center was commonly a cavity, filled with hard clay or Lithomarga.” (translated from Swedish, courtesy of Stig M. Bergström)

Linnaeus gave another description of Swedish Agnostus pisiformis fossils in his 1751 book Skånska Resa [Travels in Scania]. He likened the disarticulated heads & tails to compressed peas. The species name pisiformis is Latin for “pea-shaped”.

In 1768, Linnaeus formally named these fossils using the system of scientific names that he invented in the 1750s. Linnaeus referred to this fossil using the genus-species-subspecies name Entomolithus paradoxus pisiformis (“pea-shaped paradoxical stone insect”).

Göran Wahlenberg described & illustrated this species (as "Entomostracites pisiformis") in his 1818 monograph “Petrificata Telluris Svecanae”. Wahlenberg is often given credit for naming the species, although Carl Linnaeus was the first to name it in 1768.

This is Wahlenberg’s (1818) figure of Agnostus pisiformis from the Alum Shale Formation (Cambrian) of Västergötland, Sweden. The original figure is inverted (Wahlenberg misidentified the head as the tail & vice versa). The cephalon (head) has been designated as the lectotype specimen for Agnostus pisiformis (Wahlenberg, 1818, plate 1, figure 5). The original specimen still exists (as PMU Vg. 819, housed at the Palaeontological Museum of the University of Uppsala, Sweden), and was figured by photograph in Reyment (1976). Wahlenberg himself collected this material on 8 July 1817 from Alum Shale Formation exposures at Hönsäter, Västergötland, Sweden.

Wahlenberg's original diagnosis of this species: "caecus hemisphaericus marginatus; fronte teretiuscula"

References cited (arranged chronologically):

Bromell, M. 1729. Lithographiæ Svecanæ, specimen II, sectio II, De animalibus fossilibus, illorumque variis partibus petrificatis, caput primum, de lapidibus insectiferis & tubulis vermicularibus, articulus primus, de lapidibus insectifieris Scanicis & Gothicis. Acta Literaria Sveciæ 2: 493-497, 524-533.

Bromell, M. 1740. Mineralogia et Lithographica Svecana. Stockholm and Leipzig. G. Kiesewetter. 148 pp.

Linnaeus, C. 1747. Wästgöta Resa. Stockholm. Lars Salvius. 303 pp. 5 pls.

Linnaeus, C. 1751. Skånska Resa. Stockholm. Lars Salvius. 467 pp.

Linnaeus, C. 1768. Systema Naturæ per Regna Tria Naturæ, Secundum Classes, Ordines, Genera, Species, cum Characteribus & Differentiis, Editio Duodecima, Reformata, Tomus III, Regnum Lapideum. Stockholm. Lars Salvius. 256 pp. 3 pls.

Wahlenberg, G. 1818 (dated 1821). Petrificata telluris Svecanae. Nova Acta Regiæ Societatis Scientiarum Upsaliensis 8: 1-116, pls. 1-4.