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.