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