Evolution of metazoans

Question 1

Cambrian through the Cretaceous - overview

Answer 1

Cambrian (542 Ma) – ‘explosion’ of diversity, followed by mass extinction

Ordovician (488 Ma) – spectacular radiation of marine invertebrates. Animal life on the sea floor or burrowing in marine sediments followed by a mass extinction

Silurian (444 Ma)– marine life rebounded from end-of-Ordovician mass extinction. Swimming invertebrates. First terrestrial arthropods (mostly large millipedes, centipedes and scorpions)

Devonian (416Ma) – radiation of corals and cephalopod molluscs. Earliest radiation of fishes. Terrestrial invertebrates included centipedes, spiders, pseudoscorpions, mites and insects. The first amphibians are colonising land.

Carboniferous (318 Ma) – diversification of terrestrial invertebrates snails, scorpions, centipedes and insects. Winged insects evolved. First reptiles evolved.

Permian (299 Ma) – most modern groups of insects present. Reptiles diversify.

Triassic (251 Ma) – invertebrate lineages diversify. Many burrowers. Further reptilian radiation especially dinosaurs.

Jurassic (200 Ma) – Further radiation of fishes, reptiles and the first mammals.

Cretaceous (146 Ma) – extensive diversification of marine invertebrates.

First flowering plants evolved – and with them the coevolution of invertebrate pollinators

Question 2

Burgess Shale Fauna (mid-cambrian)

Answer 2

See this book:
Wonderful Life, The Burgess Shale (Canada) and the Natural History by Stephen Jay Gould

see also:
https://naturalhistory.si.edu/research/paleobiology/collections-overview/burgess-shale

see notes for phylogenetic tree used to base this lecture

Question 3

Metazoa evolved from colonial single celled flagellated protozoans:

Answer 3

Flagellated protozoans can be individual or exist in colonies as in the algal volvox which forms ‘daughter colonies’

Question 4

The simplest group of metazoa are Porifera

Answer 4

They are still in a sense colonial but have multiple cell types

Question 5

We will consider this lecture in Hennig’s style

Answer 5

‘Cladistic’ taxonomic classification based on evolutionary history, developed by Will Hennig in 1950.

Terminology:

Apomorphy = new form

Plesiomorphy= a trait shared between species because they share an ancestral species

Autapomorphy = alone on one lineage

Synaporphy = affects multiple lines

Homoplasy = features similar but not due to common descent (convergent evo)

Question 6

Next most simple life form after Porifera are Radiata – comprised of two orders, the Cnidarians and the Ctenophores.

Answer 6

initially had just a polyp stage then lineages evolved a medusa stage and most kept both stages some only expressed medusa stage.
Both larval and adult stages of development show radial symmetry. Among the Cnidarians, the Anthozoans (corals and anemones) are thought to be the oldest, and have only a polypoid life history stage. A medusoid stage evolved later, and includes the group we recognise as ‘jellyfish’, the Scyphozoa.
some adult Cnidaria are sessile polyps (anenomes) and some are moble medusae (jellyfish) all possess stinging cells

Martindale et al. (2002) suggest that Cnidarians and Ctenophores actually do have a primitive form of symmetry (‘biradial symmetry”), and therefore that the main synapomorphy distinguishing Radiata from Bilateria would be the coelom* (Bilateria have it, no Radiata do). Many see the symmetry Of Bilateria as distinct from that proposed for Radiata.

*No radiata have a body cavity (coleom) whereas all bilatera do. Radial symmetry is not the same as bilateral symmetry

Question 7

Bilateria

Answer 7

This is a very large group, many of which have evident bilateral symmetry in the adult form (e.g. Lobster). Some, however, are clearly not bilateral as adults (e.g. sea urchins and other Echinoderms), but they are bilaterally symmetrical in the larval stage.

Bilateria can be first divided into Protostomes and Deuterostomes

key differences are in the early stages of development.
All Bilateria have a coelom, but it forms differently in these two groups.

Protostomes – spiral formation – mouth formed first

Deuterostomes – radial formation – anus formed first

An alternative division can be made between the senu stricto Deuterostomia and the rest of the Bilateria (sensu lato Deuterostomia) plus the Protostomes, in turn divided into the Lophotrochozoa (including Annelids, Molluscs and others), and the Ecdysozoa (including Arthropods, Nematodes and others).

Question 8

Ecdysozoa

Answer 8

This is a very large group comprised of the very diverse Arthropoda (insects, spiders and crustaceans) together with Nematoda, Rotifera, Onychophora, and several other groups. All of the animals in this Phyla have a cuticle – an outer layer or organic material that functions as a skeleton. Many members of the group periodically shed their cuticle in a process called ‘ecdysis’, hence the name of the broader group.

Question 9

Lophotrochozoa

Answer 9

The Lophotrochozoa are comprised of two broad groups, the Lophophores and the Trochophores, hence the compound name (lopho+trocho). The Trochophores are perhaps especially surprising as a group, since they include molluscs and annelids (e.g. worms and squid), which as adults seem very different. However most share a similar larval form called the ‘trochophore larvae’, shown on the lower right. The larvae have two bands of cilia around the centre and a set of flagella at the top and bottom.
all share a similar larval form despite very varied adult forms

Question 10

Lophophores

Answer 10

The Lophophores (Brachiopods, Phoronids & Bryozoans) all share an unusual feeding appendage with hollow tentacles, called the ‘lophophore’ - a complex structure incorporating the second of three divisions of the coelom (to generate the hollow tentacles), with the mouth located within a ring of tentacles, and the tentacles are covered with cilia. Since this is a complex structure unlikely to evolve independently, it is seen as a synapomorphy. These are sensu lato Deuterostomes.
Many species can retract the whole lophophore into the body cavity

Question 11

Tornaria Larva are the larval form of echinoderms and hemichordata

Answer 11

see notes for diagram: On the left is the Tornaria laval form, showing the three coeloms (‘triceolomate’). On the right is an evolved version that represents an early stage of development for Echinoderms. In this case the coeloms have merged, but their separate origin is still apparent.

Question 12

Echinoderms (Tornaria larva and exoskeleton)

Answer 12

The endoskeleton is ancestral in the Echinoderms, found in all members of the phyla. It may consist of a set of interconnecting plates (as in the Asteroidae shown below) or separate ossicles, covered by epidermis on the outside.

The Echinoderms include five classes, with the Crinoidae thought to be the earliest radiation of the group. The illustration from Paul & Smith (1984, Biol. Rev. 59, 443-481) includes fossil ancestral forms at the deeper nodes.

Question 13

Echinoderm: Crinoidae

Answer 13

Crinoidae have and endoskeleton comprising hard plates in common with some later radiations within the lineage,
and tube feet like the Asteroidae.

Question 14

Echinoderm: Holothuriadae

Answer 14

Holothurians can be very small sometimes microscopic.
Feel like a slug as the exoskeleton is embedded in connective tissue.

Holothuriadae have and endoskeleton like all echinoderms, but in some cases it may be comprised Of microscopic ossicles embedded in connective tissue.

Question 15

Echinoderm: Echinoiadae (sea urchins and sand dollars)

Answer 15

Whole body encased in a hard exoskeleton
Echinoidae — sea urchins and sand dollars

Question 16

Echinoderm: Ophiuroidae (brittle stars and basket stars)

Answer 16

A very diverse group
Ophiuroidae is a large group (1600 species) found in most marine habitats around the world. It is comprised Of the brittle stars (Ophiurida ) and the basket stars (Euryalida).

Question 17

Echinoderm: Asteroidae

Answer 17

Asteroidae are the familiar sea stars, often with 5 legs, but can have many. Use tube feet for locomotion and for prising apart bivalve prey.

Question 18

Hemichordata have Tornaria larva and collar stomachords

Answer 18

Stomachord runs up the midline of the animal with a collar – as seen in earthworms
The Hemichordata (acorn worms) have a stomochord
surrounded by a collar. They are also Deuterostomes with
Tornaria larvae.

Question 19

Pikaia gracilens is a chordate from the Burgess shale, and probably our earliest ancestor

Answer 19

It is not a vertebrate, and only about 1.5 inches long in most fossils, but shows characteristics Of the chordata, including indications Of a notochord.

Question 20

Essential nature of Burgess Shale

Answer 20

It was only possible to discover Cambrian radiation due to the Burgess Shale and if this hadn’t been found it is possible that we still wouldn’t know about the diversity of this era.

Question 21

Summary

Answer 21

1) Metazoan radiation begins with the Cambrian species radiation, with many of the current groups already represented at that time (including the Chordates – us). Synapomorphy is now less relied upon as genetic analysis improves accuracy.

2) A series of division points define the radiation, starting with the division between the Radiata and the Bilateria. The Radiata (Cnidarians and Ctenophores) have radial symmetry throughout their life history (also lack a coelom), while Bilateria have bilateral symmetry at some life history stage (and all have a coelom).

3) The next division is between the Deuterostoma (subdivided into sensu stricto and sensu lato groups), and the Protostoma. The coelom forms differently in these groups (and cell cleavage is spiral in Protostomes and radial in Deuterostomes).

4) A different major division is between the Ecdysozoa (which have a cuticle) and the Lophotrochozoa, which can be further divided according to their larval stage and feeding appendage. Groups Sensu lato Deuterostomes with Protostomes.

5) Note that morphological or life history traits are used to define synapomorphies that determine these lineages, but that the synthesis of molecular and morphological data has refined our interpretation of which characters are most important in defining lineages.