Bilateria

Common ancestor

The hypothetical most recent common ancestor of all Bilateria is termed the 'urbilaterian'. The nature of this first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff, Elie Metchnikoff, Libbie Hyman, or Luitfried von Salvini-Plawen [nl]). This means that the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla (flatworms and gastrotrichs) have secondarily lost their body cavities.^[10][11] This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899.^[12] Variations of the Archicoelomata hypothesis are the Gastraea by Ernst Haeckel in 1872^[13] or Adam Sedgwick, and more recently the Bilaterogastrea by Gösta Jägersten [sv],^[14] and the Trochaea by Claus Nielsen.^[15]

One proposal, by Johanna Taylor Cannon and colleagues, is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.^[16] An alternative proposal, by Jaume Baguñà and colleagues, is that it may have resembled the planula larvae of some cnidarians, which unlike the radially-symmetric adults have some bilateral symmetry.^[17] However, Lewis I. Held presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).^[18]

Bilaterians, presumably including the urbilaterian, share many more Hox genes controlling the development of their more complex bodies, including of their heads, than do the Cnidaria and the Acoelomorpha.^[19]

Fossil record

The first evidence of Bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.^[21] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.^[22][23] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.^[24] Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old.^[25] However, more recent evidence shows these fossils are actually late Paleozoic, not Ediacaran.^[26]

Phylogeny

The Bilateria are now by far the most successful animal lineage, with over 98% of known animal species.^[27] The group has traditionally been divided into two main lineages or superphyla.^[28] The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, molluscs, and flatworms. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognise at least two more superphyla among the protostomes, Ecdysozoa^[29] and Spiralia.^[29][30][31] The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the Gnathifera.^[32][33][34]

The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence supported a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha), and the remaining bilaterians.^[28][35][36] The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004).^[28] The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.^[28][37] Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the xenoturbellids, and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.^[37]

A cladogram for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below.^[38]

A different hypothesis is that Ambulacraria is sister to Xenacoelomorpha, together forming Xenambulacraria. Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or, as shown here, to Chordata + Protostomia).^[39] The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).^[40] A 2019 study by Hervé Philippe and colleagues presents the tree, cautioning that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".^[41] As of 2024, the issue of which hypothesis is correct remains unresolved.^[42][43]

Cladogram showing Xenambulacraria hypothesis with a paraphyletic Deuterostomia:^[44]

Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:^[44]