Asteroidea  

recent & fossil

There are approximately 1500 extant species in the Asteroidea.

A survey of asteroid nomenclature arranged by order has been compiled. Clark (1989, 1993, 1996) and Clark and Mah (2001) list accepted names as well as synonyms, otherwise invalid names, references and ranges of type localities.

Brisingida—Brisingids are deep-sea dwelling asteroids. They usually have many (6-16) long, attenuated arms which are used in suspension feeding. The Brisingida contains about 100 species in 17 genera and 6 families. A preliminary phylogeny for this order has been produced by Mah (1998).

Forcipulatida—These asteroids are distinguished by their forcipulate pedicellariae, which are generally quite conspicuous on the body surface. The Forcipulatida contains about 300 species in 68 genera and 6 families. A preliminary phylogeny for this order has been produced by Mah (2000).

Notomyotida—These are deep-sea dwelling asteroids having flexible arms with characteristic longitudinal muscle bands along the inner dorsolateral surface. The Notomyotida contains about 75 species in 12 genera and 1 family.

Paxillosida—These asteroids are considered to be somewhat infaunal in that they can bury themselves partially under sandy sediments. They are characterized by some morphological features (e.g. pointed, unsuckered tubefeet) which have been considered primitive by some (see Discussion of Phylogenetic Relationships, below). The Paxillosida contains about 255 species in 46 genera and 5 families.

Spinulosida—These asteroids have a relatively delicate skeletal arrangement and completely lack pedicellariae. No fossil spinulosids have been found. The Spinulosida contains about 120 species in 9 genera and 1 family.

Valvatida—These asteroids are quite diverse, but are often characterized by their conspicuous marginal ossicles. Definition of this group has been the most variable and the ordinal definition of many families included here has been controversial (see Discussion of Phylogenetic Relationships, below). The Valvatida contains about 695 species in 165 genera and 14 families.

Velatida—These asteroids typically have thick bodies with large discs and interradial depressions. Contrary to Blake's (1987) classification, molecular evidence suggests a relationship between some velatid and valvatid families (see Discussion of Phylogenetic Relationships, below). The Velatida contains about 200 species in 25 genera and 5 families.

The earliest asteroids appeared in the Ordovician (Figure 4). However, at least two major faunal transitions have occurred within the Asteroidea concomitantly with large extinction events: in the Late Devonian (Blake and Glass in Webster et. al. 1999) and in the Late Permian (Blake 1987, Gale 1987, Blake et al. 2000, Blake and Elliot 2003, Blake and Hagdorn 2003). The asteroid orders as described here contain all extant and some extinct species which have a morphology distinct from Paleozoic forms (i.e. Ambuloasteroidea; see Characteristics, Blake 1982, 1987, 1988; Gale 1987, Blake and Elliott 2003, Blake and Hagdorn 2003). The asteroid orders are thought to have appeared and diversified very rapidly (within approximately 60 million years) during the Lower and early Middle Jurassic, frustrating our understanding of ordinal relationships (see discussion below).

Figure 4: Hudsonaster sp. (USNM 40882), an early asteroid from the Ordovician.
Image copyright © Daniel B. Blake

Relationships among Paleozoic asteroids, as well as between Paleozoic asteroids and extant asteroids, are difficult if not impossible to determine because of the limitations of the asteroid fossil record. Asteroid fossils are rare because 1) the skeletal elements rapidly dissociate after death of the animals 2) asteroids typically have a large body cavity that collapses with deterioration of the organs, resulting in misshapen forms and 3) asteroids often live on hard substrates which are not conducive to fossil formation. From the limited fossil evidence that is available we know that the basic body plan of the asteroids has remained the same since the Ordovician. Several papers by Blake (e.g 1989, 2000) describe limitations of the fossil record in detail.

Despite the paucity of the asteroid fossil record, fossil evidence has aided our understanding of asteroid evolution within both the Paleozoic and post-Paleozoic groups. One unique fossil fauna is that from the Hunsrück Slate of Germany from the Lower Devonian. These Paleozoic forms are well preserved and show a variety of morphologies. The diversity exhibited in this faunal representation suggests that the diversity of life habits of Paleozoic asteroids was probably very similar to what we see today in modern species (Blake 2000). Fossil members of the post-Paleozoic fauna have also been found. The oldest known neoasteroid is the extinct Triassic genus Trichasteropsis (Blake and Hagdorn 2003, Figure 5). Blake (1987) recognized a new order, Trichasteropsida, to contain this taxon. The slightly younger Triassic genus Noriaster barberoi, diagnosed to the extant family Poraniidae (Valvatida), is the oldest-known fossil species belonging to a surviving family (Blake et al. 2000, Figure 5).

Figure 5: Early neoasteroids from the Triassic. Images copyright © Daniel B. Blake

Left: Trichasteropsis weissmanni (MHI 843/1), Trichasteropsida. Center: Trichasteropsis weissmanni (SMNS 3173/5), Trichasteropsida. Right: Noriaster barberoi (MPUM 8420), Valvatida: Poraniidae