Hemiaster

Echinoderms are benthic marine animals found at all ocean depths, and whose first fossil traces date back to the Cambrian. They currently group five classes: starfish (Asterides), sea urchins (Echinides), sea cucumbers (Holothurians), Crinoids and Brittle stars. All species are exclusively marine, no terrestrial or freshwater species are known.

Sea Urchins also called Echinoids or Echinides; are regular and / or irregular invertebrates with a spiky body, appeared in the Lower Paleozoic and after a first phase of diversification in the Paleozoic were the actors of a very intense evolutionary radiation during the Mesozoic and Cenozoic. On the basics of a reconsideration of the homologies of the body wall of the echinoderms, some major aspects of the evolution of sea urchins are presented:

the origin of the irregularity and bilateral symmetry; a radiation model, that of the spatangues; the diversification of the modes of reproduction. These examples allow us to take stock of the innovative capacities shown by sea urchins during their post-Paleozoic radiation.

Hemiaster is a genus of echinoderms of the Echinoid class, which appears in the fossil record during the Aptian period, irregular marine invertebrate, often called cardiac sea urchin, it is a feeder-detritivore of shallow and slow infaunal deposit, Sea urchins of hearts are called irregular sea urchins because they are oblong and regular sea urchins are more circular. Hemiaster belongs to the order of Spantangoida.

Fig1 : Hemiaster globulus USNM E0000723 – Dorsal

Morphology and way of life:

In life, they were covered with many moving thorns. some thorns played a protective role, while others were paddle-shaped and were used to dig into soft sediments from the ocean floor. Hemiasters, like all sea urchins in the heart, were (and modern forms still are) infauna, which means they lived in sediments. they fed by sifting particles of organic matter called trash along five petal-shaped grooves on their shells. these grooves, called ambulacral grooves, were bordered by hydraulic tubular feet which passed the detritus towards a buccal opening on the underside of the sea urchin, located in front. irregular echinoids are distinguished by a bilateral symmetry superimposed on the radial symmetry expressed by other echinoderms.

Ecology "enviro. "and age group:

The Hemiasters can be in deep and shallow open marine environments, offshore shelf, coastal, carbonate, in the transition zone / lower shore face, basal (silicoclastic), underwater fan, estuary / bay, lagoon, perireef or subreef, terrestrial, reef, accumulation or bioherm.

They have an age range: 112.6 to 3.6 Ma2.4.

Fossil :

The fossils attributed to Hemiaster date back to the Cretaceous, appeared in the fossil record during the Aptian period, and it seems to be better known as a fossil than a living animal. It is not entirely unusual for echinoderms. Hemiaster was more diverse in the past than it is today: of the seven subgenres recognized in Hemiaster by Fischer (1966), only the nominotypic subgenre survives so far, and none of the others date of the Paleocene.

Fig3 : Hemiaster fourneli DESHAYES in AGASSIZ & DESOR, 1848

Stratigraphic distribution:

• Pliocene of Holy See (State of the Vatican City) (1 collection)

• Miocene of Australia (5), Austria (1), Egypt (1), Italy (4)

• Australian Oligocene (3)

• Eocene of Australia (1), Cuba (2), India (1), Pakistan (1), United Kingdom (5)

• Paleocene to Eocene of Australia (1)

• Paleocene from Austria (2), Denmark (1), Egypt (1), France (1), Libya (1), Spain (1), United States (6: Alabama, New Jersey)

• Paleogenic Cretaceous from the Netherlands (4)

• Cretaceous from Algeria (1), Antarctica (1), Belgium (8), Brazil (7), Canada (1: British Columbia), the Czech Republic (1), Egypt (23), France (47), Germany (3), Greenland (1), India (1), Israel (1), Jamaica (5), Japan (2), Madagascar (5), Mexico (11 ), Mozambique (4), Netherlands (6), Oman (2), Peru (6), South Africa (1), Spain (27), Switzerland (1), Tunisia (1), Turkey (1) , USSR (2), United Arab Emirates (1), United States Kingdom (5), United States (28: Alabama, Arizona, California, Mississippi, New Jersey, New Mexico, Tennessee, Texas)

Hemiaster taxonomy:

The genus includes more than 280 species, being the first to appear in the Hemiaster Oriens record and living five species today, having been a genus of great development during the Upper Cretaceous, The genus Hemiaster only exists because its Holanthus subgenus exists, according to WoRMS and Treatise.

Kingdom

Animale

Embranchement : (Phylum)

Echinodermata

Echinozoa (Subphylum)

Class :

Echinoidea

Euechinoidea (Subclass)

Irregularia (Infraclass)

Atelostomata (Superorder)

Order :

Spatangoida