Introduction to Metazoa and Porifera



Yellow slime mold from Wikipedia
Most of the semester remaining will address the fossil record of Metazoa (animals.) Metazoans are one of five groups of multicellular eukaryotes to arise during the Neoproterozoic. Unlike simple cell colonies like Volvox or slime molds (right), each of these groups forms organized bodies in which: Critters of "slime-mold grade" (right) show the starting point of this transition, being independent cells that come together to produce fruiting bodies from which spores (haploid reproductive cells) are dispersed.

The earliest fossil evidence for colonial eukaryotes comes from the Proterozoic:

These were forerunners to the emergence of multicellularity during the Cryogenian among. For the rest of the course we concentrate on Metazoa and Viridiphytae.

Metazoa

Five major groups form the main branches of its cladogram: The pattern of metazoan phylogeny is controversial. Results of molecular phylogenetic analysis are sensitive to the portion of the genome sampled (see Nosenko et al., 2013) The cladogram at right is a consensus. Ongoing controversy surrounds the positions of:

Porifera - the sponges:

(Ediacaran - Quaternary) The sponges represent the most primitive metazoan grade, lacking distinct tissues, but possessing: Sponge cells that have been mechanically separated can often coalesce into new sponges, but are unable to survive for long independently.


Sponge schematic from Steven Carr Genetics, Evolution, and Molecular systematics Lab
- Memorial University of Newfoundland

Body plan:

In their simplest form, sponges are hollow cylinders of living tissue, attached to the substrate at one end and open at the other. Key features:


Sponge cell schematic from Wikipedia

Specialized cell types:

Whether sponges lack tissues depends on one's definition. Indeed, the beginnings of distinct tissues are discernible: Sponges are capable of only the simplest organized movement, although pinacocytes can often contract in a coordinated manner, and some sponges possess specialized myocytes that can contract to close off vulnerable areas like the osculum.


Sponge organizational grades (choanocytes in red) from AuxBulles.com

Fractal architecture:

Although all sponges conform to this body plan, many increase their complexity by repeating it in a fractal manner. In the schematic at right, choanocyte-endoderm is highlighted in red:


Sponge larve from Palaeos

Reproduction may be:

Major traditional taxonomic groups (clades?)


Is Porifera monophyletic?

An unsettled question. It's generally thought that the answer is "no," but that answer takes more than one form. Complicating factors include:

This issue is far from settled. Stay tuned!


Coronacollina acula from Oak Redge National Laboratory

The First Sponges:

Whatever their phylogeny, sponges may be the most ancient metazoans. Claims for precambrian sponge fossils include:

Before we get too excited, Antcliffe et al. 2014 reject these claims. Their conservative approach identifies the earliest sponges as hexactinellids from the earliest Cambrian of Iran.

Problematic fossil poriferans (?):


Additional reading:

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