GEOL2317: Frontiers in palaeontology

Classroom exercise: Phylogenetic thinking

The adjacent figure is a schematic representation of relationships between the deuterostomes – our own subdivision of the animal kingdom.

You may not be familiar with all of these phyla. The questions below will help you to learn how to read and navigate this phylogenetic tree.

Use the colours to draw on the canvas, or clear the canvas by double-clicking the white ×.

More on chordate relationships: Satoh et al. 2014 Chordate evolution and the three-phylum system. Proc. R. Soc. B 281: 20141729.

Silhouettes: Phylopic contributors

Use the tree to identify at least four characters possessed by hemichordates.

A clade (also termed a monophyletic group) is a set of organisms comprising a single common ancestor and all of its descendants.

On the image, mark the node corresponding to the single most recent common ancestor of echinoderms (starfish and crinoids).
Mark all regions of the tree that correspond to descendants of this most recent common ancestor.

True or false: Cephalochordates, Vertebrates, Urochordates and Hemichordates together form a clade.

True

False

Hint: What is the last common ancestor of these taxa?

True or false: Echinoderms and Hemichordates together form a clade.

True

False

Mark the chordates (the group comprising cephalochordates, vertebrates and urochordates) on the diagram. Identify at least four characters present in all chordates.

Which of these characters are only found in chordates?

Characters that are unique to – and thus define – a given group are termed synapomorphies.

According to the cladogram, which evolved first?

Pharynx

Tunic

Gill slits

Sterom

Can't tell

According to the cladogram, which evolved first?

Pharynx

Tunic

Stereom skeleton

Can't tell

Which deuterostomes do not have a pharynx?

Cephalochordates

Vertebrates

Urochordates

Hemichordates

Starfish

Crinoids

Which deuterostomes have a brain?

Cephalochordates

Vertebrates

Urochordates

Hemichordates

Starfish

Crinoids

Locate the point on the tree at which the brain evolved. Which two phyla are descended from this individual?

True or false: cephalochordates are ancestors of vertebrates

True

False

Can't tell from tree

Where on the tree is the most recent common ancestor of cephalochordates and vertebrates?

This common ancestor was itself a chordate (the first crown-group chordate), but was no more a cephalochordate than a vertebrate. Both lineages have since undergone ~520 million years of evolution since being descended from this shared common ancestor, and these two independent evolutionary histories are what made each group what they are today.

See Misinterpretations about relatedness

True or false: cephalochordates are older than vertebrates

True

False

Can't tell from tree

The tree doesn't contain any absolute time information. We can say that the cephalochordate lineage diverged from the vertebrate lineage before vertebrates diverged from urochordates – but notice that this is a different question. We don't know what other lineages might have diverged from the cephalochordates before the group attained its modern identity.

As the age of a crown group is defined by the age of its most recent common ancestor, we'd need to date the divergence of all living cephalochordate lineages in order to establish the age of the crown-group phylum.

See Misinterpretations about time

Which group is most closely related to urochordates?

Cephalochordates

Vertebrates

Urochordates

Hemichordates

Starfish

Crinoids

Echinoderms

Starting at the urochordates, trace a path to the first node (branching point). What other taxa are descended from this common ancestor?

Which group is most closely related to hemichordates?

Cephalochordates

Vertebrates

Urochordates

Hemichordates

Starfish

Crinoids

Echinoderms

Starting at the hemichordates, trace a path to the first node (branching point). All echinoderms are descended from this common ancestor, so are equally closely related – just as a grandmother is equally closely related to all of her grandchildren.

Remember that you can rotate any node in the tree without changing the relationship information it contains: mirroring the position of starfish and crinoids would not change their last common ancestor, hence does not affect the relationships portrayed by the tree.

Which of these groups is the most evolved?

Cephalochordates

Vertebrates

Urochordates

Hemichordates

Can't say

This is a bad question: all the groups are alive today, so each has been evolving for 3 456 million years. Some groups share more of this history than others, depending on the sequence in which the lineages diverged.

Review: Misinterpretations about progress

The important point here is that the shape of the tree depends on which taxa are included; cephalochordates are no 'less evolved' than vertebrates, even if they exhibit a different (and less complex?) set of morphological features.

Groups that are morphologically simple may be secondarily so (as with adult urochordates); groups that are less diverse are not necessarily less sophisticated.

Which one of these trees shows a different set of relationships to the others?

Which group is most closely related to urochordates? To vertebrates?

Remember, the most closely related group is the group that shares a most recent common ancestor. Common ancestors occur at the branching points on a path traced towards the root of the tree.

Gyaltsenglossus is a Cambrian fossil with a three-part body, comprising a proboscis, collar and trunk.

Where on the tree is it most likely to plot?

Unless the three-part body denotes convergent evolution, it belongs in the Hemichordate total group. The information available is insufficient to determine whether it belongs in the crown or stem group.

Banffia is another enigmatic Cambrian fossil from the Burgess Shale; it has a feeding appendage, but lacks a stereom or three-part body.

Use stem-group and crown-group terminology (refresher) to describe its most parsimonious phylogenetic positions.

Refresher: Building trees with parsimony

Banffia could be a stem-group hemichordate, a stem-group echinoderm, or a stem-group ambulacrarian. (Ambulacraria is the clade comprising echinoderms (starfish, crinoids and relatives) and hemichordates.)

Another problematic Cambrian fossil: Rotadiscus has a robust upper surface that has some similarity to the urochordate tunic. It lacks a brain, skull, stereom, three-part body, notochord or tail; and has gill slits and feeding appendages. Its other attributes are uncertain.

Mark its most parsimonious phylogenetic position on the tree.

What makes this position unparsimonious? What evolutionary scenario might account for the combination of characters observed?

If the tunic is homologous with that of urochordates, Rotadiscus would be a member of that total group. We would have to account for the absence of a brain, notochord and tail – unless, as with modern urochordates, these three characters were present in larval stages and lost in the fossilized adults. We'd also have to interpret the feeding appendages as convergent traits, leaving a total of four extra steps on our tree.

It is more parsimonious to treat the potential tunic as a homoplasy (convergently derived character), in which case the other characters combine to position Rotadiscus in the stem lineage of Hemichordates and/or echinoderms.

The full story is interesting to explore!

Nectocaris is another Burgess Shale fossil; it has been interpreted as having a muscular tail, but lacks a notochord.

Mark its possible phylogenetic position on the tree.

If hearts, brains and notochords are quick to decay, their absence could reflect non-preservation rather than an original biological absence.

Mark the tree to indicate the possible position of Nectocaris if the original presence of these organs is equivocal.

If the notochord is genuinely absent, the affinity of Nectocaris can be constrained to quite a precise position in the chordate stem lineage – and Nectocaris would help to constrain the sequence of evolution (i.e. a muscular tail preceded the evolution of the notochord).

However, if the absence of these characters is taphonomic, there is much more uncertainty about its position: it could also sit higher in the stem lineage, or in the chordate crown group, with an affiliation to one or more chordate phyla.

(In fact, the interpretation of Nectocaris has since been substantially revised, and it is no longer thought to be a deuterostome.)

GEOL2317: Frontiers in Palaeontology

Welcome to the Frontier

What we’ll do

Before the first session

Classroom exercise: Phylogenetic thinking