Naraoiidae

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Naraoiidae
Naraoia spinosa

Naraoia spinosa

Temporal occurrence
Cambrian (2nd series) to Silurian ( Pridolian )
520 to 410 million years
Systematics
Primordial mouths (protostomia)
Molting animals (Ecdysozoa)
Arthropod (arthropoda)
Order : Nectaspida
Naraoiidae
Scientific name
Naraoiidae
Walcott , 1912

The Naraoiidae are an extinct trilobite-like family of arthropods that have only survived in fossil form and that mainly occurred in the lower and middle Cambrian .

description

The Naraoiidae include animals whose bodies were covered in life by two large, articulated plates ( tergites ), so that only these could be seen when viewed from the dorsal point of view. As a result, they resemble aberrant trilobites in appearance , which would only be made up of a head shield and tail shield with no free segments in between. The anterior (front) or head shield was usually slightly wider than the posterior (rear) or tail shield, but this was noticeably longer. The arched shields in life had little differentiated structures on the outside, in some species there is a laterally directed row of thorns on the side edge, or at the rear end of the anterior shield there are two so-called cheek thorns at the side of the joint. In contrast to the trilobites, however, the dorsal shell of the exoskeleton was not reinforced by calcium deposits. The body structure ( Tagmose ) of the Naraoiidae was markedly different on the dorsal (upper) and the ventral (lower) side.

In numerous fossil specimens, a tree-shaped branched structure in two parts symmetrical to the center line can be seen on the head shield, which either forms a relief or is colored differently, mostly darker, by taphonomic mineral precipitates. According to the unanimous interpretation, this is the digestive system of the animals, which consists of a central intestine, from which numerous, multi-branched, blind-ended diverticula or caeca branched off laterally , the foremost branch being by far the largest and filling almost the entire anterior shield . A straight, almost smooth intestinal tube with only a few, weakly pronounced side branches can be seen in the rear end. According to the anatomy and the intestinal contents preserved in fossil form, it is likely that the Naraoiidae were substrate eaters, ate the nutrient-rich, organically enriched mud and digested the organic content. But some researchers also adopt a predator or scavenger lifestyle.

While such fossils of Naraoiidae with relatively few features, which only show the two dorsal plates, can be common and widespread in the corresponding strata, there are also findings from the underside of the body (ventral side) of significantly fewer specimens. Here the animals showed a radically different appearance. The two tergites are opposed by a long row (up to 30), significantly narrower sternites . Most of them, between 15 and 26, sat under the posterior shield. The body begins on the underside at the front end with a head flap called a hypostome, which appears in three parts in the Naraoiidae with three protrusions. The lateral protrusions are sometimes interpreted as complex eyes . Long, articulated antennae were attached to the hypostome and protruded far beyond the head shield. The antenna portion below the head shield is very seldom preserved, in a single specimen of Misszhouia longicaudata it can be seen that the base of the antenna whip was a tiny, tripartite minor whip; so possibly the antennas go back to split legs . In the segments behind the antenna-bearing section (the segment nature of which is subject to interpretation and is therefore controversial), one pair of extremities each corresponded to a sternite, similar to the recent euarthropods. The legs were constructed very similarly over the length of the body, the first pair of legs already corresponded to the basic construction plan. Real mouthparts are therefore not recognizable, although the foremost pair of legs sat to the side of the hypostome and the mouth opening. The front part of the body with the head shield always carried four pairs of extremities in addition to the antennae.

The leg structure of the Naraoiidae can be seen in almost all species. They are typical split legs . Proximal to the body sat a plate-like member called the base. An endite (or gnatho base) covered with thorns was set off on the inside, the endite of both rows of legs encompassed a central food channel. It can therefore be assumed that, as is the case with the recent gill-pods , for example , all legs were involved in food intake in addition to locomotion. The actual extremity was attached to the base. The inner branch (endopodit) of the split bone consisted of seven limbs in all species and a detached claw at the end. The structure of the exopodite differed between the species. Usually it was pinnate in the shape of a comb, with a base shaft bearing a row of flat, ribbon-like lamellae, at the end of the row sat a flap-like appendix with numerous bristles. In Naraoia spinosa the exopodite was closed tongue-shaped, which became wider towards the outside. The structure of the leg is completely similar to that of original trilobites such as Eoredlichia or Olenoides , but also other trunk group arthopods such as Xandarella . The fact that both comb-like and tongue-shaped exopodites were present in species of a genus shows that the trait can only be used with caution for a phylogenetic analysis.

distribution

Naraoiidae are reported in the fossil record from the Cambrian , Ordovician, and Silurian . Most of the best preserved Cambrian finds come from the Burgess Shale of British Columbia and the Chengjiang Faunal Community from the Maotianshan Shale of the People's Republic of China . Other finds come from the Central Cambrian Marjum Formation in Utah, USA, and Gibson Jack Formation in eastern Idaho, USA. The Ordovician Pseudonaraoia hammanni was found in the Middle Ordovician Šárka Formation, on the Red Mountain (Červený vrch) near the city of Prague . The only Silurian representative, Naraoia bertiensis, comes from the Upper Silurian ( Pridolium ) Bertie Formation in southeastern Ontario .

Systematics and phylogeny

Eight species from three genera are assigned to the family Naraoiidae:

  • Naraoia
    • Naraoia compacta Walcott, 1912. Type species of the genus.
    • Naraoia spinifer Walcott, 1931
    • Naraoia halia Simonetta & Delle Cave, 1975 (the species is regarded by some taxonomists only as a synonym of Naraoia compacta )
    • Naraoia spinosa Zhang and Hou, 1985
    • Naraoia bertiensis Caron et al., 2004
    • Naraoia taijiangensis Peng et al., 2012
  • Misszhouia Chen et al., 1997
    • Misszhouia longicaudata (Zhang & Hou, 1985).
  • Pseudonaraoia Budil et al., 2003
    • Pseudonaraoia hammanni Budil et al., 2003

The Naraoiiden belong to a fossil group that has some features in common with the more well-known trilobites , but also has marked differences compared to them, most noticeably the non-calcified dorsal shell and the complete lack of free trunk segments. They are traditionally placed with numerous other fossil arthropods with similar combinations of characteristics, mostly from the Cambrian, in a broad, poorly defined family circle, which is referred to as Lamellipedia, Trilobitomorpha or Arachnomorpha. Workers in the past even assigned the Naraoiidae as an order to the trilobites themselves, at a time when most of the related groups were still unknown. Similar fossil forms have been found several times in recent years, for example Arthroaspis bergstroemi from the Sirius Passet Fauna Community of Greenland in 2013 or the genera Emucaris and Kangacaris from the Lower Cambrian Emu Bay slate from Kangaroo Island, South Australia.

According to recent phylogenetic analyzes on a cladistic basis, the most likely sister group of the Naroiidae is the family Liwiidae , a species-poor and poorly researched group, which was only newly described in 1988, which are very similar to the Naroiidae in most characteristics, but, unlike them, have free trunk segments. Closest related to this clade would be the Emucarididae of Australia (now also given from China). These groups together are understood as the order Nectaspida Raymond, 1920 (also written Nectaspia, Nectaspidida or Nektaspida). Their togetherness is now considered well secured.

While the relationship is still quite plausible up to this point, the further classification is still quite speculative and varies between different scientific studies. The group could form a clade often called Artiopoda with some others, including the trilobites, or, with the inclusion of Marrella and relatives (Marrellomorpha), a taxon Lamellipedia. According to many, but not all analyzes, this group is probably more closely related to the stem group of the mandibular animals (Mandibulata) than to that of the jaw-claw carriers (Chelicerata). The previously favored summary as "Arachnomorpha" appears today only one possibility among many.

Naraoia compacta

swell

  • X.-L. Zhang, D.-G. Shu, DH Erwin (2007): Cambrian naraoiids (Arthropoda): morphology, ontogeny, systematics, and evolutionary relationships. Journal of Paleontology 81: 1-52. doi : 10.1666 / 06-082.1

Individual evidence

  1. ↑ Brigitte Schoenemann, Euan NK Clarkson (2012): Compound Eyes in the Chengjiang Biota. Scientific Papers University of Latvia, Earth and Environmental Sciences 783: 57-71.
  2. ↑ Jean-Bernard Caron, David M. Rudkin, Stuart Milliken (2004): A New Late Silurian (Pridolian) Naraoiid (Euarthropoda: Nektaspida) from the Bertie Formation of Southern Ontario Canada: Delayed Fallout from the Cambrian Explosion. Journal of Paleontology 78 (6): 1138-1145.
  3. ↑ Jin Peng, Yuanlong Zhao, Hijing Sun (2012): Discovery and significance of Naraoia from the Qiandongian (lowe Cambrian) Balang Formation, Eastern Guizhou, South China. Bulletin of Geosciences 87 (1): 143-150.
  4. ^ P. Budil, O. Fatka, J. Bruthansová (2003). Trilobite fauna of the Šárka Formation at Praha - Červený vrch Hill (Ordovician, Barrandian area, Czech Republic). Bulletin of Geosciences 78 (2): 113-117.
  5. ↑ a b Martin Stein, Graham E Budd, John S Peel, David AT Harper (2013): Arthroaspis n. Gen., A common element of the Sirius Passet deposit (Cambrian, North Greenland), sheds light on trilobite ancestry. BMC Evolutionary Biology 2013, 13:99. on-line
  6. ↑ a b J.R. Paterson, GD Edgcombe, DC Garcia-Bellido, JB Jago JG Gehling (2010): Nektaspid arthropods from the Lower Cambrian Emu Bay Shale deposit, South Australia, with a reassessment of lamellipedian relationships. Palaeontology 53: 377-402. doi : 10.1111 / j.1475-4983.2010.00932.x
  7. ↑ Martin Stein & Paul A. Selden (2011): A restudy of the Burgess Shale (Cambrian) arthropod Emeraldella brocki and reassessment of its affinities. Journal of Systematic Palaeontology 10 (2): 361-383. doi : 10.1080 / 14772019.2011.566634
  8. ↑ David A. Legg, Mark D. Sutton, Gregory D. Edgecombe (2013): Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4: 2485. doi : 10.1038 / ncomms3485
  9. ^ Plate from Charles D. Walcott. Cambrian Geology and Paleontology / Volume 2 / Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata, 1912 full text in Wikisource

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