Introduction to Eumetazoa and Cnidaria
Eumetazoa:(Ediacaran - Quaternary) Includes Cnidaria, Ctenophora, and Bilateralia.
- Cnidaria: (Ediacaran - Quaternary) corals, jellyfish, etc.
- Ctenophora: Comb-jellies, with a poor fossil record, although interesting specimens are known from the Burgess Shale (Conway Morris and Collins, 1996). Shu et al, 2006 have proposed connections between ctenophores and frond-like Ediacaran organisms.
- Bilatera: (Ediacaran - Quaternary) Animals with bilateral symmetry, and distinct mouth and anus.
- Most analyses place Placozoa within Eumetazoa or as its sister-taxon, although a few data sets put them closer to Porifera. Placozoans are bizarre, minute (1 mm width), simple creautres with no fossil record (although it is informally suggested that the Ediacaran weirdo Dickinsonia might be an early placozoan.) They were first discovered living in salt-water aquaria, and their wild ecology is unknown.
Gastrulation from Wikipedia
- Distinct tissues including:
- Body organization in which distinct germ layers:
- Endoderm: lining the gut cavity
- Ectoderm: covering the outer surface
Cnidarian body plan from College of DuPage BIO1151
- Diploblastic - having endoderm and ectoderm only, separated by gelatinous extracellular mesoglea.
- Radially symmetrical body plan (right) with single opening to the gut, termed the "mouth" for the sake of politeness. Cnidarians have no front, back, left, or right, but we can refer to their oral and aboral aspects.
- Prey capture by Cnidocytes (stinging cells) born on tentacles surrounding mouth.
Cnidocyte from Boundless
- Epitheliomuscular cells forming organized muscle fibers in both ectoderm and endoderm.
- Nerve cells, including sensory and motor neurons, organized into a decentralized nerve net.
- Gland cells of the endoderm secrete digestive enzymes.
- Interstitial cells - undifferentiated totipotent cells that can transform, as needed, into other cell types.
- Cnidocytes (right) containing nematocysts capsules - the stinging apparatus. The nematocyst stores Ca2+ ions. When triggered by a mechanical disturbance, these are released into the cytoplasm, creating an osmotic gradient that causes water to flow into the cell, inflating the capsule and extending the barb.
Cnidarian life cycle from siera104/com
- In sexual reproduction, the zygote develops into a planktonic gastrulated planula larva that settles down to develop in to a polyp.
- Asexual reproduction can take the form of:
- Fission: where single polyps divide in two then regenerate lost parts.
- Budding: New individuals bud off of parents. Polyps routinely produce medusae in this way. In some (E.G. hydrozoans) budding is off of the side, in others (Scyphozoans) it occurs along the body axis at the oral end (right).
Major cnidarian groups:
Sea nettle - Chrysaora quinquicirha
- Body plan shows four-fold radial symmetry.
- Although generations alternate in most studied taxa, the polyp stage is reduce and medusa predominates. Medusae bud off of the oral ends of polyps (in contrast to hydrozoans.)
- Reliable record begins with the Late Ediacaran (~550 Ma) Bjarmia cycloplerusa (Grazhdankin, 2016). The Middle Cambrian (~505 Ma) Marjum Fm. of Utah (Cartwright et al., 2007) offers convincing scyphozoan fossils as well..
- Claims of Ediacara hills (Ediacaran) scyphozoans are controversial, as the fossils (E.G. Mawsonites) are impressions that might represent the holdfasts of other organisms.
Fossil record: Restricted to Konzervatlagerstätten such as Burgess Shale (Cambrian), Holtzmaden (Jurassic), and Solnhofen (Jurassic).
Box Jelly from Vista al Mar
- Body plan shows four-fold radial symmetry with four tentacles or groups thereof.
- Medusa profile is square when viewed from above.
- Four eyes, complete with retinas and corneae, (organs!) facilitate more active pursuit of prey and obstacle avoidance.
- Medusae appear to practice internal fertilization. Polyp generation reproduces asexually, but eventually, rather than budding medusae, polyps undergo a metamorphosis to become sexually reproducing medusae.
- Restricted to two Konservat-Lagerstätten, Mazon Creek (Carb) and Solnhofen (J).
Millepora tenella from Wikipedia
- Marine and fresh-water. The freshwater Hydra is typical.
- Generations altrnate, but medusa stage is reduced. Polyps may reproduce asexually by budding from the side of the polyp (in contrast to the scyphozoan pattern.)
- Solitary or colonial. Marine colonial forms include blade fire coral (right), which secretes calcareous skeleton.
- Hyrdoida: (Paraphyletic by all accounts) Solitary or colonial unspecialized polyps.
- Milleporina (right) and Stylasterina (hydrocorals): Marine colonial forms include fire coral, which secrete calcareous skeleton.
- Siphonophora: Pelagic (free-swimming) colonial marine forms, such as Portuguese Man-o-War, showing radical specialization of zooids (both polyp and medusa in one colony) into:
- Pneumatophores: Bell or sail for floatation.
- Nectophores: For swimming.
- Gastrozoids: For feeding.
- Milleporids and stylasteroids known from Cretaceous onward.
Major groups: Here we must simply punt. Hydrozoan systematics are in a state of profound disagreement and revision. The following are interesting hydrozoan ecomorphs whose monophyly is entirely uncertain:
Lucernaria quadricornis from Marine Flora and Fauna of Norway
- Body plan shows four-fold radial symmetry with four tentacles or groups thereof.
- Lack polyp generation.
- Medusae are stalked.
Fossil record: Either:
Sea-anemone from Dormivigilia
- Medusa stage is totally suppressed.
- Typically secrete some sort of hard coralite.
- Solitary or colonial. In colonial forms, regardless of nature of coralite, individual polyps are connected by sheets of living tissue.
- Colonial, typically secreting durable framework of protein gorgonin.
- In some, gorgonin is lost and polyps sprout from matrix of soft tissue - soft corals.
- Polyps have strict eightfold radial symmetry with there being exactly eight tentacles, rather than merely multiples of eight - hence name.
- Common colony morphs include:
- Interpretation of most frond-shaped Ediacaran forms as sea-pens is very problematic.
- The frond-shaped Burgess Shale taxon Thaumaptilon walcotti is actually thought to have housed zooids and may be a genuine sea pen.
- Pywackia baileyi (Middle Cambrian), long thought to be the first bryozoan, was reinterpreted as an octocorallian (Taylor et al., 2013.)
- Each major group seems, independently to have descended from a soft-bodied ancestor that achieved the ability to secrete a calcareous skeleton.
- Hermatypic members of each group hosted photosynthesizing zooxanthellae, enabling them to grow and secrete CaCO3 much more rapidly. These tend to be the major reef-formers. They are restricted to the photic zone.
- Ahermatypic members lack zooxanthellae and tend to be solitary and less abundant, but can colonize below the photic zone.
- Corallites of the major zoantharian groups display the same characteristic features but in different combinations (see right):
- Septa: Vertical partitions projecting from the sides that support pleated infoldings of the polyp, that increase the surface area of its gut. Different taxa have distinct patterns of septum growth visible in the calyx (aperture of the corallite). Plural = calices. Note: "calice" is not a word!
- Tabulae: As the polyp grows it moves upward in the conical corallite, then secretes a new floor to support it. The result is a series of horizontal sheets perpendicular to the long axis of the corallite.
- Dissepiments: Small upwardly convex "bubbles" of CaCO3 that fine-tune the shape of the corallite.
- Generally solitary, some colonial.
- Corallite composed of calcite.
- Display prominent septa, often also tabulae and dissepiments.
- Characteristic sequence of septum growth (right) yields calyx with distinct cardinal and alar fossulae.
- Largely ahermatypic, not major reef framework builders, but crucial pioneers of soft sediments, as their skeletons provided hard substrates for later framework builders.
- Significant component of Ordovician-Devonian tabulate-rugosan-stromatoporoid reefs.
- Suffered greatly in Devonian extinction. Persisted at lower abundance and diversity to the Permian extinction.
- Paraphyletic with respect to Scleractinia? Possible, although equally likely that scleractinian skeleton is convergent.
- Major reef formers. Geochemical analysis by Zapalski, 2013 indicate that they employed zooxanthellae. Thus hermatypic.
- Corallite composed of calcite.
- Appear to have required a hard substrate, so tended to appear later in reef ecological succession.
- Always colonial.
- Individual polyps small, occupying tubular corallites with numerous tabulae but never septa or dissepiments.
- Major component of Ordovician-Devonian tabulate-rugosan-stromatoporoid reefs.
- Greatly reduce by Devonian extinction.
- Extinguished by Permian extinction.
- Predominantly colonial, although solitary forms exist (E.g. Fungia.)
- Distinguished from rugosans by six-fold symmetrical septal pattern (right). (Hence old name "hexacorals.")
- Attach to substrate by a basal plate, and can secrete aragonite onto outer surface of corallite, facilitating solid attachment and growth of large colonies (in contrast to rugosans.)
- In living members, the ability to form large reefs depends on zooxanthellae. Thus, major reefs limited to photic zone of tropics. Major reef building corals (with zooxanthellae), thus hermatypic.
- In many scleractinians, the polyp and calyx, in oral view, becomes greatly elongate, with colony members forming convoluted interlocking patterns. E.g. "brain-coral" (right).
- Corallite composed of aragonite.
- First unambiguous appearance in Middle Triassic.
- Minor reef formers in Late Triassic and Jurassic, however their fortunes waned in the Cretaceous, when the major reef-building was done by rudistid bivalves.
- But bounced back after the K-P extinction as the Primary reef-builders of Cenozoic.
- Calcite seas: During intervals of rapid sea floor spreading, hydrothermal activity near spreading zones pump Ca into the oceans, but tend to withdraw Mg through hydrothermal reactions with ocean bedrock. Thus, oceans are Mg-poor and primary deposition is of low-Mg calcite. These also tend to be intervals of higher sea level.
- Aragonite seas: During intervals of slow sea floor spreading, oceanic concentrations of Ca are lower and Mg are higher, favoring aragonite and high-Mg calcite deposition. These tend to be intervals of lower sea levels.
Gorgonians at Wee Wee Cay, Belize
Fossil record: The octocorallian fossil record is very poor.
Rugose coral corallite schematic from Paleontological Laboratory at SUNY Cortland
Rugose coral calyx from Paleontology and Geology of Missouri
Tabulate coral from Kentucky Paleontological Society
Evolutionary trends: Similar to those of Rugosa.
Grooved brain coral - Diploria labyrinthiformis
Calcite vs aragonite seas from Wikipedia
Scleractinian origins: All skeleton-secreting zoantharians probably derived from soft-bodied forebears like the Early Cambrian Archisaccophyllia of Chengjiang (Han et al., 2010). Thus, their early history is encrypted. In the case of scleratinians, we know of rare Paleozoic anthozoans with the scleractinian septal pattern, including Kilbuchophyllum (Ord). Unlikely that later calcified scleractinians are derived from it. More likely, Kilbuchophyllum testifies to the existence of the scleractinian septa pattern in soft anthozoans of the time, but represents an independent derivation of calcification.
A cloudy picture. The last common ancestor of Porifera (or is it Homoscleromorpha?) and Cnidaria is difficult to visualize. Calibrated molecular clocks suggest:
- Cryogenian (~660 ma) LCA for Metazoa
- Early Ediacaran (~630 ma) LCA for Eumetazoa
- Latest Ediacaran/ Earliest Cambrian (~543 ma) LCA for Cnidaria
Some tantalizing hints:
- Namapoikea, a meter-scale modular colonial organisms of the Ediacaran Nama Formation of Namibia.Whether this was closer to sponges or cnidarians is completely unclear.
- Cloudina (right), calcareous tubes of internested narrow cones found worldwide in the Ediacaran. The presence of some with budding sturctures leads to speculation that they were cnidarians.
Even the ancestral condition of cnidarians is problematic. Some questions that arise:
- Was the ancestral cnidarian colonial or solitary?
- Was it nektonic or sessile?
- Did it experience alternation of generations?
- If not, was it a polyp or medusa?
- Did polyps bud asexually in the manner of hydrozoans or scyphozoans?
- Did it have fourfold symmetry (as in cubozoans, scyphozoans, and staurozoans) eightfold (as in Octocorallia) or none?
Available information is ambiguous:
- The oldest fossils seem to be of solitary forms, but the most basal branch, Anthozoa, is primarily colonial.
- If we assume that Ediacaran scyphozoan and conulariid fossils are legitimate, then both free and attached forms were present throughout the Phanerozoic.
- The oldest fossils seem to be medusae, but the most basal branch, Anthozoa, has only polyps.
- But for conulariids, with fourfold symmetry, there is no clear pattern of symmetry.
Cambroctoconus orientalis from Nature Communications
- Colonial with a hard skeleton (the earliest known from the cnidarian stem or crown.)
- Consisting of polyps in which individual polyps bud asexually off of their parents from the side, like modern Hydra (or Cloudina), not like scyphozoans.
- Possess septa but no dissepiments and at most one tabula.
- Display striking eightfold symmetry.
- Hard parts are of apatite (calcium phosphate)
- Geometry resembles a tall, tapering Chinese restaurant carry-out box with:
- Four sides
- Flaps at the top
- Ribbed texture resulting from growth by addition of apatite rods.
- Occasionally found with ectopic "pearls" of apatite.
- Exceptionally preserved individuals appear to show:
- Tentacles at the wide end
- A holdfast at the narrow end.
- Typical conulariids appear in Late Cambrian.
- Nevertheless, conulariid-like fossils are known from the Ediacaran including
Relationships?: Two sustantive hypotheses:
- Staurozoa , Simoes and Marques, 2004 see them as closer to Staurozoa based on four-fold symmetry and general similarity to staurozoan medusa.
- Scyphozoa, Van Iten et al. 2006 regard conulariids as close to the Coronatae within Scyphozoa.
- Vendobiont based on the arguable presence in the Ediacaran. That these early conulariids (if that's what they are) have three-fold symmetry undermines the argument for a relationship with Cnidarians. Could they be independently derived from other three-fold symmetric Ediacarans like Tribrachidium?
- Paulyn Cartwright, Susan L. Halgedahl, Jonathan R. Hendricks, Richard D. Jarrard, Antonio C. Marques, Allen G. Collins, Bruce S. Lieberman. 2007. Exceptionally Preserved Jellyfishes from the Middle Cambrian. PLOS|one, October 31, 2007.
- Alexander G. Liu, Jack J. Matthews, Latha R. Menon, Duncan McIlroy, Martin D. Brasier. 2014. Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B, 281(1793).
- Tae-yoon Park, Jusun Woo, Dong-Jin Lee, Dong-Chan Lee, Seung-bae Lee, Zuozhen Han, Sung Kwun Chough, and Duck K. Choi. 2011. A stem-group cnidarian described from the mid-Cambrian of China and its significance for cnidarian evolution. Nature Communications, 2(442).
- D.-G. Shu, S. Conway Morris, J. Han, Y. Li, X.-L. Zhang, H. Hua, Z.-F. Zhang, J.-N. Liu, J.-F. Guo, Y. Yao, K. Yasui. 2006. Lower Cambrian Vendobionts from China and Early Diploblast Evolution. Science 312(5774): 731-734.
- Heyo Van Iten, Juliana de Moraes Leme, Marcello Simoes, Antonio Marques, and Allen Collins. 2013. Reassessment of the phylogenetic position of conulariids (?Ediacaran - Triassic) within the subphylum medusozoa (phylum cnidaria). Journal of Systematic Paleontology 4(2).
- Heyo Van Iten, Juliana de Moraes Leme, Antonio Marques, and Marcello Simoes. 2013. Alternative interpretations of some earliest Ediacaran fossils from China. Acta Palaeontologica Polonica 58 (1): 111–113.