Despite their fragile, gelatinous bodies,
fossils thought to represent ctenophores – apparently with no tentacles but many more comb-rows than modern forms – have been found in
Lagerstätten as far back as the early
Cambrian, about . Nevertheless, a recent molecular phylogenetics analysis concludes that the common ancestor originated approximately 350 million years ago ± 88 million years ago, conflicting with previous estimates which suggests it occurred , shortly after the
Cretaceous–Paleogene extinction event.
Fossil record Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in
Lagerstätten, places where the environment was exceptionally suited to the preservation of soft tissue. Until the mid-1990s, only two specimens good enough for analysis were known, both members of the crown group, from the early
Devonian (Emsian)
period. Three additional putative species were then found in the
Burgess Shale and other Canadian rocks of similar age, about in the mid-
Cambrian period. All three lacked tentacles but had between 24 and 80 comb rows, far more than the eight typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. The youngest fossil of a species outside the crown group is
Daihuoides from the late Devonian, which belongs to a basal group that had been assumed to have gone extinct more than 140 million years earlier. The Ediacaran
Eoandromeda could putatively represent a comb jelly. It has eightfold symmetry, with eight spiral arms resembling the comblike rows of a ctenophore. If it is indeed ctenophore, it places the group close to the origin of the Bilateria. The early Cambrian
sessile frond-like fossil
Stromatoveris, from China's
Chengjiang lagerstätte and dated to about , is very similar to
Vendobionta of the preceding
Ediacaran period. De-Gan Shu,
Simon Conway Morris,
et al. found on its branches what they considered rows of cilia, used for
filter feeding. They suggested that
Stromatoveris was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system. Other Cambrian fossils that support the idea of ctenophores having evolved from sessile forms are
Dinomischus,
Daihua,
Xianguangia and
Siphusauctum which also lived on the seafloor, had organic skeletons and cilia-covered tentacles surrounding their mouth, which have been found by
cladistic analysis as members of the ctenophore
stem-group 520 million-year-old Cambrian fossils also from Chengjiang in China show a now wholly extinct class of ctenophore, named "
Scleroctenophora", that had a complex internal skeleton with long spines. The skeleton also supported eight soft-bodied flaps, which could have been used for swimming and possibly feeding. One form,
Thaumactena, had a streamlined body resembling that of
arrow worms and could have been an agile swimmer. sister to the
Cnidaria,
Placozoa, and
Bilateria, and sister to all other animals.
Walter Garstang in his book
Larval Forms and Other Zoological Verses (
Mülleria and the Ctenophore) even expressed a theory that
ctenophores were descended from a
neotenic Mülleria larva of a
polyclad. A series of studies that looked at the presence and absence of members of gene families and signalling pathways (e.g.,
homeoboxes,
nuclear receptors, the
Wnt signaling pathway, and
sodium channels) suggest that ctenophores are either sister to
Cnidaria,
Placozoa, and
Bilateria or sister to all other animal phyla. Several more recent studies comparing complete sequenced genomes of ctenophores with other sequenced animal genomes support ctenophores as sister to all other animals. This position would suggest that neural and muscle cell types either were lost in major animal lineages (e.g.,
Porifera and
Placozoa) or evolved independently in the ctenophore lineage. They also have extremely high rates of
mitochondrial evolution, and the smallest known RNA/protein content of the
mtDNA genome in animals. As such, the Ctenophora appear to be a basal
diploblast clade. In agreement with the latter point, the analysis of a very large sequence alignment at the metazoan taxonomic scale (1,719 proteins totalizing acid positions) in Simion
et al. (2017) showed that ctenophores emerge as the second-earliest branching animal lineage, and sponges are sister to all other multicellular animals. Despite all their differences, ctenophoran neurons share the same foundation as cnidarian neurons after findings shows that peptide-expressing neurons are probably ancestral to chemical neurotransmitters. The issue with the "rate of evolution" counterargument is that it mainly affects analyses based on the sequence of genes, not those based on gene family presence or synteny, both of which have produced results in support of the "Ctenophora sister" theory. their genome express only a single type of
voltage-gated calcium channel unlike other animals which have three types, and they are the only known animal phyla that lack any true
Hox genes.
Innexin genes, which code for proteins used for
intercellular communication in animals, also appears to have evolved independently in ctenophores.
Internal phylogeny }} Relationships within Ctenophora (2001). A molecular phylogeny analysis in 2001, using 26 species, including four recently discovered ones, confirmed that the cydippids are not monophyletic and concluded that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps survived the
Cretaceous–Paleogene extinction event while other lineages perished. When the analysis was broadened to include representatives of other phyla, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain. A 2017 study corroborates the paraphyly of
Cydippida but finds that
Lobata is paraphyletic with respect to
Cestida. ==See also==