Aglaspidida

from Wikipedia, the free encyclopedia
Aglaspidida
Strabops

Strabops

Temporal occurrence
Cambrian to Ordovician
500.5 to 445.2 million years
Systematics
Primordial mouths (protostomia)
Over trunk : Molting animals (Ecdysozoa)
Trunk : Arthropod (arthropoda)
Sub-stem : Artiopoda
Superclass : Vicissicaudata
Order : Aglaspidida
Scientific name
Aglaspidida
Walcott , 1912

As aglaspidida refers to a fine extinct mariner (on the seabed living) arthropods (Arthropoda) of the Cambrian and Ordovician . With at least 21 genera they attained a considerable abundance of forms, but remained very rare at most of the sites where they were found, many species have only survived in a few individual copies. They died out again before the end of the Ordovician (in the Katium ). The systematic position and relationships of the Aglaspidida have been controversial for a long time. Today they are mainly classified in a clade called Artiopoda , as very close relatives of the trilobites .

features

Aglaspidida were small to medium-sized arthropods with a phosphate- containing cuticle . They have this mineralization of the exoskeleton in common with trilobites and many crustaceans (but these are calcareous). The relatively lightly mineralized shell is reinforced by the incorporation of the calcium phosphate hydroxylapatite . Comparable formations occurred in some, not closely related, Paleozoic arthropods, but are unknown in recent representatives. Due to the mineralization, only the mineralized upper side (dorsal side) of the animals remained fossil in almost all cases. The underside and the body appendages are known only from a few genera and also poorly preserved in these, so that many features are controversial or unknown here.

The body of the Aglaspidida was divided into a large head shield with a semicircular front and a trunk section of usually eleven, occasionally fewer (minimum six, in Brachyaglaspis singularis ) free, somewhat overlapping tergites . The trunk segments were laterally widened by appendages called pleurs, making the entire animal shield-shaped, mostly with a relatively closed body outline. The trunk of some species was arched in the middle section and thus, similar to the trilobites, indistinctly three-lobed, but more often uniformly arched and not articulated. Both the head shield and the pleura of the trunk segments could be extended laterally into backward-pointing spike-like tips. The last tergite formed the base of a centrally located tail spine of different lengths, which was mostly distinctly broadened at the base. By comparison with other arthropod-like physique, which had a twelfth free segment at this point (the Sidneyia found in the Burgess slate even two), it is assumed that it was the result of the fusion of the Telsons with one (or more) body segment (s ) emerged. On the ventral side, the last trunk segments carried a so-called post-ventricular plate that protruded backwards and was usually divided into two parts, which also covered the base of the tail spine. This is the most important, and the only undoubtedly confirmed, autapomorphy of the group. Also typical of almost all Aglaspidida was a reinforced edge or ridge on the upper side of the pleura, in the basal section normally hidden by the following segment, i.e. only visible on twisted or partially disintegrated specimens.

The top of the head shield was completely free of visible peeling sutures; on the underside, the hypostome was possibly delimited by such. The head section had four, or possibly five, pairs of extremities, with the first pair having a simple, antenna-like appearance (i.e. no chelicerae formed). All other extremities were single-branched (uniram) ( i.e. no split bones) and leg-like, special mouth limbs are not known. The eyes were located dorsally , mostly quite large, protruding from the contour of the head towards the back and kidney-shaped. They sat relatively far in front and close together, mostly on both sides of a central bulge ( glabella ). However, eyes are missing in many species.

Systematics and taxonomy

The systematics and taxonomy of the group are extremely confused. The group was set up by Charles Walcott for the genus Aglaspis Hall , under the name Aglaspina , in 1862 ( type Aglaspis spinifer ). Gilbert Oscar Raasch completely rejected Walcott's grouping and diagnosis and instead proposed a new, differently circumscribed order, which he named Aglaspida. Later authors named either Walcott or Raasch as the first descriptor, depending on personal preference, with Raasch at times, following Stephen Peter Hesselbo , while the newer editors again preferred Walcott. In 1968, Jan Bergström coined the new name Aglaspidida as an emendation, which has caught on, although it was not absolutely necessary according to the nomenclature rules.

The Aglaspidida were originally assigned to the (presumably not monophyletic) "Merostomata". They were mostly regarded as "lower Xiphosura " (these are also paraphyletic according to current knowledge) and thus counted among the jaw-claw carriers (Chelicerata). This classification lost credibility after Derek Briggs and colleagues showed in 1979 that the first pair of extremities of Aglaspis spinifer were not, as previously thought, formed as chelicerae . Later adaptations, for example by Hou & Bergström, Chen et al. 2004 and Scholtz & Egdecombe 2005 brought further progress. The inclusion of species of unclear affiliation such as Kwanyinaspis maotianshanensis, Zhang and Shu, which, according to later findings, do not belong to the Aglaspidida, turned out to be problematic . The basic for today's interpretation was the cladistic analysis by van Roy 2006, which was later updated, mainly due to newly discovered species. Current revisions are from Ortega-Hernández and colleagues and Rudy Lerosey-Aubril and colleagues.

The Aglaspidida therefore belong to a group called Artiopoda. This included the majority of the Cambrian arthropods, in addition to the trilobites as the most famous group, for example the little-known Cheloniellida, Conciliterga, Nektaspidida and Xandarellida. Their relationship with the jaw-claw carriers (Chelicerata) is still unexplained and controversial. If, as most editors assume, the Chelicerata do not belong, the Artiopoda is a completely extinct sub-strain of the arthropods. Within the Artiopoda, the Aglaspidida form a common clade with the little-known Cheloniellida and some isolated species of unclear affiliation, including the best-known representative Emeraldella brockii , which has been described as Vicissicaudata. In the past, the entire group, which includes a number of poorly preserved fossil species with few characteristics, of unclear affiliation, was summarized as "Aglaspidida-like arthropods". Presumably the Aglaspidida evolved from ancestors with a morphology similar to the genera Emeraldella and Sidneyia, which have been handed down from Burgess slate .

The following genera are assigned to the Aglaspidida (after)

incertae sedis (without family assignment): Australaglaspis Ortega-Hernández et al., Beckwithia Resser.

The following are uncertain about affiliation: Angarocaris Černyšev, Chacharejocaris Černyšev, Girardevia Andreeva, Obrutschewia Černyšev, Rozhkovocaris Rosov.

Occurrence

The oldest finds of Aglaspidida come from the Guzhangian : the genera Tremaglaspis from Wales and Utah and Beckwithia from Utah and Wisconsin, with the oldest finds in the Laurentia Shelf Sea . The greatest diversity was achieved in the Jiangshanium . The youngest known representative is Chlupacaris from the Katium of Morocco. It is noteworthy that there is no evidence whatsoever from the older Cambrian strata after the Cambrian explosion , although there are numerous well-researched deposits with a species-rich arthropod fauna from this period.

The group is distributed almost worldwide, with finds from Europe (Great Britain), Africa, North America, Siberia, East Asia (China) and Australia (Tasmania).

swell

literature

  • Jason A. Dunlop , Paul A. Selden : The early history and phylogeny of the chelicerates . In: RA Fortey & RH Thomas (eds.): Arthropod relationships . Systematics Association Special Volumes Series, No. 55. London: Chapman & Hall, 1997: 221-235. PDF
  • P. van Roy: An aglaspidid arthropod from the Upper Ordovician of Morocco with remarks on the affinities and limitations of Aglaspidida . Transactions of the Royal Society of Edinburgh: Earth Sciences 96, 2006: 327-350. PDF

Individual evidence

  1. ^ GO Raasch: Cambrian Merostomata . Special Papers of the Geological Society of America 19, 1939: 1-146.
  2. ↑ Javier Ortega-Hernández, Peter Van Roy, Rudy Lerosey-Aubril (2015): A new aglaspidid euarthropod with a six-segmented trunk from the Lower Ordovician Fezouata Konservat-Lagerstätte , Morocco. Geological Magazine 153 (3): 524-536. doi: 10.1017 / S0016756815000710
  3. ↑ James C. Lamsdell (2013): Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura. Zoological Journal of the Linnean Society 167: 1-27. doi: 10.1111 / j.1096-3642.2012.00874.x
  4. ^ L. Størmer: On the relationships and phylogeny of fossil and Recent Arachnomorpha. A comparative study on Arachnida, Xiphosura, Eurypterida, Trilobita and other fossil Arthropoda . Skrifter utgitt av Det Norske Videnskaps-Akademi i Oslo. I. Matematisk-Naturvidenskapelig Class 5, 1944: 1–158.
  5. ^ L. Størmer: Merostomata . In: Moore, RC (Ed.): Treatise on Invertebrate Paleontology . Part P. Arthropoda 2, 1955: 4-41.
  6. ^ DEG Briggs, DL Bruton, HB Whittington: Appendages of the arthropod Aglaspis spinifer (Upper Cambrian, Wisconsin) and their significance . Palaeontology 22, 1979: 167-80. PDF
  7. ↑ Hou X.-G., J. Bergström: Arthropods of the Lower Cambrian Chengjiang fauna, southwest China . Fossils & Strata 45, 1997: 1-116.
  8. ↑ Chen J.-Y., D. Waloszek, A. Maas: A new 'great-appendage' arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial antero-ventral appendages . Lethaia 37, 2004: 3-20.
  9. ^ G. Scholtz, GD Edgecombe: Heads, Hox and the phylogenetic position of trilobites . In: S. Koenemann, RA Jenner (Ed.): Crustacea and arthropod relationships . Crustacean Issues 16, 2005: 139-65. Boca Raton, Florida: CRC Press.
  10. ^ Peter van Roy: An aglaspidid arthropod from the Upper Ordovician of Morocco with remarks on the affinities and limitations of Aglaspidida . Transactions of the Royal Society of Edinburgh: Earth Sciences 96, 2006: 327-350. doi: 10.1017 / S0263593300001334 PDF
  11. ↑ Javier Ortega-Hernández, David A. Legg, Simon J. Braddy (2013): The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda. Cladistics 29: 15-45. doi: 10.1111 / j.1096-0031.2012.00413.x
  12. ↑ a b Rudy Lerosey-Aubril, Xuejian Zhu, Javier Ortega-Hernández (2017): The Vicissicaudata revisited - insights from a new aglaspidid arthropod with caudal appendages from the Furongian of China. Scientific Reports 7, Article number: 11117 doi: 10.1038 / s41598-017-11610-5 (open access)

Web links

Commons : Aglaspidida  - collection of images, videos and audio files