Strabops

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This article is about the extinct strabopid. For the beetle, see Strabops (beetle).

Strabops
Temporal range:
Ma
S. thacheri type specimen exhibited at the Peabody Museum of Natural History, Connecticut
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Order: †
Strabopida
Family: †Strabopidae
Genus: †Strabops
Beecher, 1901
Type species
†Strabops thacheri
Beecher, 1901

Strabops is a

aglaspidids with uncertain affinities. The generic name is composed by the Ancient Greek
words στραβός, meaning "squinting", and ὄψῐς, meaning "face" (and therefore, "squinting face").

The history of Strabops has been turbulent and confusing since its original description by

eyes
.

Description

Restoration of S. thacheri

As the other

strabopids, Strabops was a small-sized arthropod, measuring only 11 centimetres (4.3 inches) in length.[1] However, it was the largest of the strabopids, surpassing Paleomerus (9.3 cm, 3.7 in)[2] and Parapaleomerus (9.2 cm, 3.6 in).[3]

Like some other arthropod groups, the strabopids possessed

mineralized (with inorganic substances),[6] sturdy and calcareous (containing calcium). The head of the strabopids was very short, the back was rounded and lacked trilobation (being divided into three lobes), the abdomen was composed by 11 segments and was followed by a thick tail-like spine, the telson.[7][4]

In the genus Strabops, the prosoma was short and broad, with a rounded outline. The

eyes were located in the middle of the front of the prosoma. These were medium-sized, ovate and narrow, and pointed obliquely inwards (hence the name Strabops). Two spots between the eyes indicate the presence of the ocelli (light-sensitive simple eyes). In its abdomen, there were eleven segments, being the third the widest. The ends of the segments were rounded on the sides. In the posterior part of the segments, a row of tiny crenulations was visible. The first six segments were uniform in size, the three following ones were somewhat shorter and the last two were the longest. The telson was a broad, flat spine, and it rose slightly in the middle.[1] The appendages are unknown, although it has been suggested that they be less than seven pairs (what has been considered an overestimate).[8]

Strabops differed only from Paleomerus in the position of the eyes, which were closer together and farther from the margin than in Paleomerus,[6] and the size of the telson, being longer and narrower than in the latter.[8]

History of research

Size comparison of Strabops and both species of Paleomerus

Strabops is known by one only well preserved specimen (YPM 9001, housed at the

lateral.[10]

In his book Cambrian Merostomata of 1939, the American paleontologist and

Aglaspida.[11]

In 1971, the Swedish geologist and paleontologist Jan Bergström tentatively removed Strabopidae (at that time containing Strabops and

Derek Ernest Gilmor Briggs et al. in 1979 has shown that Aglaspis spinifer had between four and five pairs of appendages, but not six, weakening Bergström's argument.[12]

In 1997, Bergström and

apomorphies (distinctive characteristics), such as the lack of genal spines (a spine placed in the posterolateral part of the prosoma).[9] Other authors have reinforced this argument by the trapezoidal telson form of Paleomerus and Strabops in contrast to the long styliform telson of the aglaspidids.[8] However, some authors prefer to represent the taxonomic position of the strabopids as uncertain due to the poor preservation of their fossils.[13]

Classification

Reconstruction of the closely related Paleomerus hamiltoni. Some authors suspect they may represent synonyms.

Strabops is classified in its own order, Strabopida,[14] in the clade Arachnomorpha,[8] along with Paleomerus, Parapaleomerus and potentially Khankaspis.[14] It was described originally as the only Cambrian eurypterid,[1] and later as an aglaspidid.[11] It would not be until 1997 when the order Strabopida was described,[7] but there is still doubt if the exclusion of them from Aglaspidida was really correct. The current status of the strabopids is of aglaspidid-like arthropods of uncertain affinities.[13]

Strabops shares with the other strabopids a series of characteristics that distinguish them all from the other arthropods. These are an abdomen divided into eleven segments followed by a thick spine, the telson. The head was short with sessile compound eyes. The back was rounded. Like Paleomerus, Strabops possessed prominent

dorsal eyes, however, there is no evidence of this in the fossils of Parapaleomerus.[7][14]

The great similarity that Strabops and Paleomerus share has cast doubt on many authors about whether both genera are really synonymous or not. The Norwegian paleontologist and geologist Leif Størmer described Paleomerus as an intermediate form between Xiphosura (commonly known as horseshoe crabs) and Eurypterida, only highlighting a unique feature different from Strabops, a twelfth segment.[2] Nevertheless, a fourth specimen found in Sweden has shown that this extra segment actually represented the telson of the animal,[8] making them virtually indistinguishable.[9] Although this should convert both genera into synonyms, over time, more differences have been highlighted, such as the position of the eyes (closer to each other and farther from the margin in Strabops than in Paleomerus)[6] and the size of the telson (longer and narrower in Strabops than in Paleomerus), which keeps them as separate but closely related genera.[8]

The

Scorpiones and other arachnid clades) and their outgroup taxa (used as a reference group). Strabops and Paleomerus are shown as the sister taxa of Aglaspida.[9]

Aglaspida

Strabops

Paleomerus

Lemoneites

Weinbergina

Bunodes

Kasibelinurus

Xiphosurida

Eurypterida

Scorpiones

Other arachnids

Note that there are several outdated elements. For example, Lemoneites was remitted to the Glyptocystitida order of echinoderms in 2005.[14]

Paleoecology

The type and only known specimen of Strabops has been found in Furongian (Upper Cambrian) deposits in eastern Missouri.[1] Strabops was at least an inhabitant of the sea, if not born in it. In addition, there are two specimens of the marine brachiopod Obolus lamborni and a poorly preserved trilobite head attached to the slab.[15]

References

  1. ^
    doi:10.2475/ajs.s4-12.71.364
    .
  2. ^ a b Størmer, Leif (1956). "A Lower Cambrian merostome from Sweden" (PDF). Arkiv för Zoologi. Series 2. 9: 507–514.
  3. S2CID 88949935
    .
  4. ^
    ISBN 9781118896310
    .
  5. ^ Størmer, Leif (1955). "Merostomata". Treatise on Invertebrate Paleontology, Part P Arthropoda 2, Chelicerata. p. 23.
  6. ^
    doi:10.1111/j.1502-3931.1971.tb01862.x
    .
  7. ^
    ISBN 9788200376934
    .
  8. ^
    S2CID 129086749
    .
  9. ^
    ISBN 978-94-010-6057-8
    .
  10. ^ Clarke, J. K., Ruedemann R. (1912) "The Eurypterida of New York"
  11. ^
    ISBN 9780813720197
    .
  12. ^ Briggs, Derek Ernest Gilmor; Bruton, David L.; Whittington, Harry Blackmore (1979). "Appendages of the arthropod Aglaspis spinifer (Upper Cambrian, Wisconsin) and their significance" (PDF). Palaeontology. 22: 167–180.
  13. ^
    S2CID 85744103
    .
  14. ^
    S2CID 83671216
    .
  15. PMID 16586669
    .
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