Until now, the earliest deuterostomes scientists have discovered were between 510 and 520 million years old, when they had already begun to diversify into vertebrates, sea squirts, echinoderms such as starfish and sea urchins, and hemichordates
(Saccoglossus kowalevskii), echinoderms (Strongylocentrotus purpuratus), and some vertebrates (see Table 2 for GenBank sequence accession numbers for the data in Fig.
Thus, the objectives of this study were to make accessible data on the abundances of sipunculans, brachiopods and hemichordates
(1984-1987, 2013) at a sand-mud flat; and on trace metals (1996, 2000) and abundances (2015) of sipunculans and brachiopods at a sand flat in the upper Gulf of Nicoya estuary.
The chordates (that include vertebrates), hemichordates
(that include the living "acorn worms"), and echinoderms (that include the living starfish and echinoids) are all deuterostomes and have the same pattern of early embryo development.
Acorn worms are themselves part of the hemichordates
, a group of marine animals closely related to today's sea stars and sea urchins.
There is much interest in these deep sea hemichordates
since they are close to the evolutionary link between vertebrates and invertebrates.
This animal is thought to share affinities with both echinoderms (a group that includes the starfish) and a small group of marine animals called hemichordates
, but the exact connection remains difficult to establish.
Although the two phyla appear quite different today, chordates are often linked with the hemichordates
and another dissimilar phylum, the echinoderms, which include starfish.
Nonfeeding larvae have evolved from feeding larvae many times within the echinoderms, hemichordates
, lophophorates, polychaetes, gastropods, bivalves, crustaceans, and other taxa (Strathmann 1978).
Acorn worms (Enteropneusta), a solitary class of hemichordates
, are marine vermiform invertebrates that generally live concealed in burrows (Hyman, 1959).
are a type of marine animal split into two groups: the acorn worm, which live in burrows, and the pterobranchs, which live in a tubular structure anchored to the ocean floor.
A group of 15 species included flatworms, nemerteans, two sipunculans, oligochaetes, two brachiopods, two echinoderms, hemichordates
, a lancelet and a gobiid fish (Vargas 1989).