Introduction to Eumetazoa and Cnidaria


(Ediacaran - Quaternary) Includes Cnidaria, Ctenophora, and Bilateralia.

Major taxa:

Gastrulation from Wikipedia
Synapomorphies: We now turn to the eumetazoans with substantial fossil records.


Cnidarian body plan from College of DuPage BIO1151
(Ediacaran - Quaternary) Body plan: One typically hears that cnidarians have tissues but not organs. Typically true, except that gametes are produced in proper gonads. A few other scattered exceptions noted below.

Cnidocyte from Boundless
Major cell types: Cell types among cnidarians are no more diverse than in sponges, but are clearly organized into distinct tissues. These include:

Cnidarian life cycle from siera104/com
Reproduction: Cnidarians typically alternate between asexually reproducing polyp and sexually reproducing medusa generations (although in various groups, one or the other of these generations may be brief or absent.)

Major cnidarian groups:

Sea nettle - Chrysaora quinquicirha
Scyphozoa: Jellyfish (Ediacaran - Quaternary)

Box Jelly from Vista al Mar
Cubozoa: Box jellies (Carboniferous - Quaternary)

Millepora tenella from Wikipedia
Hydrozoa: Hydras, fire coral (Cretaceous - Quaternary)

Lucernaria quadricornis from Marine Flora and Fauna of Norway
Staurozoa: Stalked jellies (Ediacaran? - Quaternary)

Sea-anemone from Dormivigilia
Anthozoa: (Ordovician- Quaternary). Characteristics:

Major groups:

Cnidarian origins:

A cloudy picture. The last common ancestor of Porifera (or is it Homoscleromorpha?) and Cnidaria is difficult to visualize. Calibrated molecular clocks suggest:

Some tantalizing hints:

Inconclusive but not out of line with molecular clocks. An aside: (Penny et al., 2014) report that Cloudina in addition to being the first widespread mineralizer, was also the first reef-builder, in association with bacterial stromatolites.

Even the ancestral condition of cnidarians is problematic. Some questions that arise:

Available information is ambiguous:

Cambroctoconus orientalis from Nature Communications
Some illumination has been provided by Park et al., 2011, with the description of the Middle Cambrian Cambroctoconus orientalis (right scale is 1 cm.) Argued to be a "stem cnidarian" (i.e. the sister taxon to proper Cnidaria) it appears to be:

Carl from

Problematic fossils:

Conulariida: (Late Cam. - Early Tr.) Enigmatic fossil cnidarians with a distinctive hard skeleton.


Fossil record:

Relationships?: Two sustantive hypotheses:

Stay tuned.

Additional reading:

To Syllabus.