GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2020
Overview of Animal Phylogeny & Cnidarians

Great Barrier Reef - Queensland.

Key Points:
•Eumetazoa - metazoans closer to Bilateria than to "Porifera" includes creatures with proper endoderm and ectoderm, displaying gastruation.
•Basal eumatazoans, including Placozoa and Ctenophora have negligible fossil records.
•Cnidaria are characterized by distinct body plans, cell types, and reproductive strategies.
•Cnidarians ancestrally alternate between polyp and medusa generations, however different branches may modify this pattern, reducing or eliminating one stage or the other.
•Only the cnidarian groups whose members secrete calcareous skeletons (Hydrozoa and Anthozoa) have abundant fossil records.
•Rugosa and Tabulata are limited to the Paleozoic. They were significant elements of Silurian and Devonian reef environments, but less common in the Carboniferous and Permian.
•Scleractinia have been present since the Triassic but only came to dominate reef environments during the Cenozoic.
•The earliest definite cnidarian fossils are from the Cambrian, however Ediacaran organisms such as Namapoikea and Cloudina might be stem cnidarian.
•Conulariids are enigmatic Paleozoic organisms that have been allied with Staurozoa and Scyphozoa, but also considered not to be proper metazoans.

"The coral that grows at the edge of the reef is always the strongest and most colourful because it faces the greatest battering. It's the same if you're called Honeysuckle. I'd have had a totally different life if I'd been called Mary."
(Honeysuckle Weeks.)


(Ediacaran - Quaternary) Includes Cnidaria and Bilateria. (Many regard Ctenophora as members.)

Major taxa:

Gastrulation from Wikipedia

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 Wikipedia
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.)

Cnidarian Diversity:

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 Coolgalapagos
Anthozoa: (Cambrian- 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. They are currently a research focus of UMD's own Dr. Jay Kaufman!

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 substantive hypotheses:

Stay tuned.


Sometimes called "Bilateralia" in older literature. Monophyletic group containing most animals. In simple terms, bilaterians are animals with a distinct front, back, top, and bottom.

Fundamental synapomorphies:

Bilaterian Development:

Cleavage from A. S. Romer. 1977. The Vertebrate Body.
Consider the basic steps by which we get a proper embryo with a top, bottom, front, and back from a zygote:

Polarity: In all ova yolky matrial tends to concentrate at one end, yeilding:

The amount of yolk greatly influences developmental dynamics. For now, we consider an ovum with relatively little yolk.

Cleavage Phase of rapid cell division with little overall growth. The zygote transforms into a hollow sphere of cells, the blastula. The space in the middle is the blastocoel

The blastula has three cell types:

Gastrulation from A. S. Romer. 1977. The Vertebrate Body.

The embryo is now a gastrula. It possesses three basic germ layers: Note: This should look slightly familiar, as it is basically a cnidarian planula larva with mesoderm added.

Eucoelomate coelom schematic from A Review of the Universe
The Coelom: A characteristic feature of bilaterians is the presence of a coelom or body cavity. This feature allows for: Developed as:

Eucoelomate coelom schematic from Fueleducation
The evolution of the coelom opened many vistas for animal evolution, including significantly expanded locomotor strategies. Cnidarians show the limits of what a hydrostatic skeleton can do for an animal with a single module. Bilaterians, however, display body segmentation in which separate modules of the hydrostatic skeleton can lengthen and shorten, facilitating much more complex movement. This makes possible activities like: Indeed, non-bilaterians are deemed incapable of burrowing.

Specialized organs: These capabilities came at a price. Animals with only endoderm and ectoderm don't need to worry about gas exchange and elimination of nitrogenous waste, because no living cell is so far from the body surface that simple diffusion can't do the trick. Bilaterians, in contrast, usually require specialized organs for functions like:

Fortunately, the presence of mesoderm and a coelom seems to bestow the developmental plasticity needed to allow these to evolve. Indeed, the gut tube, kidneys, and gonads are ancestrally suspended inside the coelom.

Hox Genes:

First, fully appreciating the significance of segmentation as a synapomorphy of Bilateria, and the deep relationship shared by bilaterians requires an excursion into the realm of genomics:

Remember operons?

Hox gene clusters from The Biology Corner
Hox genes: During the late 20th century it became known that segmentation in bilaterian bodies is governed by a special class of regulatory genes. The story:

Fruit flies are a favorite model for geneticists, with short generation spans and interesting mutations that often effect entire sections of the body (modifying or eliminating body segments and/or the appendages that grow from them). Investigation into these segmentation-altering mutations revealed that they can be caused by mutations to eight genes. What makes this interesting:

The homeobox codes for a protein called the homeodomain that is functionally similar to regulatory proteins that block or allow transcription of other genes. It appears that the cluster of eight genes controls the identity of body segments in fruit flies. These are called Hox genes, after the homeoboxes they contain. The homeodomain protein regulates transcription of the Hox gene, whose protein products, themselves, promote or inhibit the synthesis of other regulatory proteins that govern body segmentation. Thus, Hox genes and the Hox gene clusters are special classes of operon. But that is the tame part!

But it gets better: The search for homologs to fruit fly Hox genes found them in almost every animal surveyed. Mammals, for example, have four clusters of Hox gene homologs, in each of which the genes occur in the same order on the chromosome as the regions for which they code.

This is huge.

The fruit flies and mammals belong to the two major clades of bilaterians, and their last common ancestor lived during (or before) the Ediacaran, and yet they share important regions of the genome.

The fruit flies and mammals belong to the two major clades of bilaterians, and their last common ancestor lived during (or before) the Ediacaran, and yet they share important regions of the genome. Moreover, two Hox genes have been identified in the cnidarian Nematostella and are involved with the patterning of its oral/aboral axis. We will meet Hox genes again, but know that they are present from the beginnings of Eumetazoa.

Additional reading:

To Syllabus.