Cephalon (arthropod head)

Cephalon (arthropod head)
Left: Peronopsis interstrictus, an Agnostida from Cambrian age strata of Utah.
Center:Praecambridium sigillum
Right:The enigmatic creature, Ainiktozoon loganese, a Thylacocephala, which was once thought to be an ancestral chordate, but is now thought to be a peculiar-looking crustacean. The trilobite is Calymene blumenbachi. Both lived in Silurian Great Britain.

The cephalon is the head section of an arthropod. It is a tagma, i.e., a specialized grouping of arthropod segments

The word cephalon derives from the Greek κεφαλή (cephale), meaning "head". It is rather used in Trilobite terminology. In insects, head is a preferred term. In arachnids and crustaceans, the cephalothorax is derived from the fusion of the cephalon and the thorax, and is usually covered by a single unsegmented carapace. In relation with the arthropod head problem, phylogeny studies show that members of the Malacostraca class of crustaceans have five segments in the cephalon, when not fused with the thorax to form a cephalothorax.

In the Late Precambrian or Lower Cambrian proarticulatan species Praecambridium sigillum, that resembles a segmented trilobite-like arthropod, the term is also used to describe the anterior part of the animal. The Agnostida, an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian, also have a cephalon.

The Thylacocephala, a unique group of extinct arthropods, with possible crustacean affinities, thought to date from the lower Cambrian, while the class has a definite presence in Lower Silurian marine communities and survived to the Upper Cretaceous, has also a cephalon.

Contents

Trilobite cephalon

The tagmata in a trilobite

Morphology of the Trilobite cephalon

1 – fixigena; 2 – librigena; 3 – glabella
1 – preocular area; 2 – palpebral area; 3 – postocular area; 4 – posterolateral projection; 5 – occipital ring; 6 – glabella; 7 – posterior area; 8 – lateral border; 9 – librigenal area; 10 – preglabellar area

The cephalon of trilobites is highly variable with a lot of morphological complexity. The glabella, the expression of the axial lobe in the cephalon, forms a dome underneath which sat the "crop" or "stomach". Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backwards at the rear edge of the hypostome.

Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described.[1] The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niches.[2]

The lateral fringe of the cephalon is greatly exaggerated in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch.[3] Highly complex compound eyes are another obvious feature of the cephalon.

Facial sutures

When trilobites moulted, the librigenae ("free cheeks") separated along the facial suture to assist moulting, leaving the cranidium (glabella + fixigenae) exposed. Trilobite facial sutures can be roughly divided into three main types (proparian, gonatoparian, and opisthoparian) according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge). Early Cambrian trilobites belonging to the suborder Olenellina (like Fallotaspis) lacked facial sutures. Other later trilobites also lost facial sutures secondarily.[4]

See also

References

  1. ^ Fortey, 1990
  2. ^ Fortey, 2004
  3. ^ Fortey, R. A.; Hughs, N. C. (1998), "Brood pouches in trilobites", Journal of Paleontology 72: 639–649, http://findarticles.com/p/articles/mi_qa3790/is_199807/ai_n8785182. 
  4. ^ Chris Clowes (April 15, 2006). "Trilobite Origins". Peripatus. http://www.peripatus.gen.nz/taxa/arthropoda/trilobita/TriOri.html. Retrieved April 13, 2011. 

Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать курсовую

Look at other dictionaries:

  • Cephalon — For information about the head portion in arthropod anatomy, see cephalon (arthropod head). Cephalon, Inc. Type Subsidiary of Teva Pharmaceutical Industries Industry Biotechnology, Biopharmaceutical Founded 1987 Headqu …   Wikipedia

  • arthropod — /ahr threuh pod /, n. 1. any invertebrate of the phylum Arthropoda, having a segmented body, jointed limbs, and usually a chitinous shell that undergoes moltings, including the insects, spiders and other arachnids, crustaceans, and myriapods. adj …   Universalium

  • cephalon — /sef euh lon , leuhn/, n., pl. cephala / leuh/. Zool. the head, esp. of an arthropod. [1870 75; < NL, for Gk kephalé head] * * * …   Universalium

  • Crustacean — Crustacea Temporal range: 511–0 Ma …   Wikipedia

  • Thylacocephala — Taxobox | name = Thylacocephala fossil range = Early Cambrian Late Cretaceous regnum = Animalia phylum = Arthropoda subphylum = Crustacea? classis = Thylacocephala classis authority = Pinna, Arduini, Pesarini and Teruzzi, 1982The Thylacocephala… …   Wikipedia

  • trilobite — trilobitic /truy leuh bit ik/, adj. /truy leuh buyt /, n. any marine arthropod of the extinct class Trilobita, from the Paleozoic Era, having a flattened, oval body varying in length from 1 in. (2.5 cm) or less to 2 ft. (61 cm). [1825 35; < NL… …   Universalium

  • protocephalon — |prōd.(ˌ)ō+ noun (plural protocephala) Etymology: New Latin, from prot + cephalon 1. : the part of an insect embryo that consists of the prostomium and the first postoral somite 2. : the primitive arthropod head correspon …   Useful english dictionary

  • Facial suture (trilobite anatomy) — Facial sutures are sutures in the cephalon (head) of most trilobites, along which the exoskeleton splits during molting. Contents 1 Description 1.1 Dorsal sutures …   Wikipedia

  • Trilobite — ] Highly complex compound eyes are another obvious feature of the cephalon (see below). Figure 3 shows gross morphology of the cephalon. The cheeks (genae) are the pleural lobes on each side of the axial feature, the glabella. When trilobites… …   Wikipedia

  • Tagmosis — is the evolutionary process of and modifying segments in metameric organisms such as arthropods to form tagmata. In insects, the body segments (somites or metameres) have become fused into three main tagmata: the head, thorax and abdomen.… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”