Agnostus
pisiformis
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
below is a black, fossiliferous packstone from the classic locality of
Andrarum, Scania, southern Sweden. These rocks are slightly
petroliferous, and release a distinctive oily smell when broken. Early
Swedish workers called these rocks “stinkstones”. The stinkstone shown
below is packed with numerous disarticulated heads & tails of the common
agnostoid trilobite Agnostus pisiformis.
Some concretions in the Alum Shale Formation have Agnostus pisiformis trilobites with
preserved soft parts. They are part of
the Orsten Lagerstätte, which includes preserved soft parts from the
larval stages of several invertebrate species.
Agnostus pisiformis (Wahlenberg, 1818) from the Alum Shale Formation (Cambrian)
at Andrarum, Sweden (YPM 36654, Yale University’s Peabody Museum, New Haven,
Connecticut, USA). Centimeter scale.
Agnostus pisiformis (Wahlenberg, 1818) from the Alum Shale Formation
(Cambrian) at Andrarum, Sweden (YPM 36654, Yale University’s Peabody Museum,
New Haven, Connecticut, USA).
Left:
Magnus von Bromell (1679-1731) was an early Swedish naturalist. He first described & illustrated Agnostus pisiformis stinkstones in 1729 .
From Reyment (1974).
Right:
Bromell’s (1729) figure of black fossiliferous packstone from the Alum Shale
Formation Cambrian) with abundant Agnostus pisiformis cephala &
pygidia.
These fossils were 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)
Carl Linné (Carolus Linnaeus) (1707-1778) From Wilson
(1994).
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.
Left:
Göran Wahlenberg (1780-1851). From Reyment (1976).
Right:
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.