This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we introduce the concepts of “crown groups” and “stem groups” and apply them to Cambrian fauna to reveal the stages that the arthropod lineage progressed through to reach the modern arthropod “body plan”.
In our last post, we introduced the concept that what we define in the present day as a “body plan” or “shared suite of characteristics of a monophyletic group” in fact presents a challenge for classifying organisms from the deep past. The reason, as we discussed, is that any modern-day suite of characteristics is not expected to have arisen all at once, but rather in a step-by-step, piecemeal fashion.
Let’s return to our discussion of arthropods to see how biologists handle this challenge in the face of real data. The general approach is to start with the characteristics of a monophyletic group of species that has at least two species (or more) represented in the present day. These species, their common ancestral species, and all descendants of that common ancestral species form what is known as a “crown group” on a phylogeny (often abbreviated as a triangle to represent numerous species). Accordingly, we can define “crown group” arthropods as the last common ancestral population of all living arthropod species and all of their descendant species (living or not). Other extinct groups that have some, but not all, of the characteristics of the crown group are then classified as “stem group” species:
The question then arises: is a “stem group” arthropod really an arthropod? Yes and no – such species are not a crown group arthropod, since they do not possess all of the characteristics that define modern-day arthropods (and their common ancestral population, and its descendants). However, they are more closely related to crown-group arthropods than to any other monophyletic group with living representatives, and they possess at least some of the characteristics of the crown group. As such they are arthropods in a sense, but better described as “stem group” species.
Let’s expand this example to include another crown group – velvet worms (Onychophora). Velvet worms share some features in common with arthropods (such as appendages and segmentation) but not others (for example, they lack the complex specialization of segments seen in arthropods).
Phylogenic analyses (using comparative morphology, the fossil record, and DNA sequencing) consistently place velvet worms as a close relative of arthropods. Since velvet worms have living species, we can define a crown group for them as well (as before, the last common ancestral population for all living velvet worm species, and all of its descendant species). As for arthropods, as we look back in the fossil record, we see some extinct species that have some, but not all, of the defining features of crown-group velvet worms. Accordingly, we can place these species on the “stem” of the velvet worm lineage:
What should be obvious from this phylogeny is that the distinction between a stem-group arthropod and a stem-group onychophoran will become blurred as the two lineages converge (moving from the present day backwards towards their common ancestral population). Species that we find in the fossil record can be assigned as a stem to either lineage based on their suite of characteristics, and whether they show closer relatedness to the arthropod or onychophoran lineage.
Stem-group arthropods in the Cambrian
With this understanding in hand, we can now turn to actual species that we observe in the Cambrian fossil record as examples of stem-group arthropods. One well-known, large Cambrian predator is Anomalocaris (literally, “abnormal shrimp”). Anomalocaris has a number of features that are clearly shared with crown-group arthropods, such as large compound eyes, specialized jointed appendages, specialized segmentation, and other features. What it lacks, however, is a hardened exoskeleton over its entire body (it has hardened appendages only). This suite of characteristics places it as a close relative to crown group arthropods (e.g. we would place Anomalocarisat the “X1” position in the phylogeny above), and also provides information about the state of characteristics present at the time that the lineage leading to Anomalocaris branched away from the lineage leading to crown group arthropods (i.e. that a fully hardened exoskeleton was one of the last characteristics that the ancestral population leading to crown-group arthropods acquired).
Other species in the Cambrian show even fewer characteristics of crown-group arthropods, but yet still exhibit at least some features in common. The bizarre group of species known as Hallucigenia is an example. Features that Hallucigenia share in common with common with crown-group arthropods are specialized appendages and specialized segmentation – but other features, such as a hardened exoskeleton (on either appendages or the body as a whole), are absent. In other words, these species are stem arthropods that are more like what we would expect of stem onychophorans.
Building body plans, step by step
Taken together, we can summarize these findings as follows:
What we observe as the emergence of a new taxonomic unit (“phylum,” “family,” “genus,” and so on) is somewhat arbitrary (since it in actuality describes a continuum) and in fact is decided only in hindsight, based on the characteristics of monophyletic groups in the present day.
For better or for worse, taxonomy has been trying to shoehorn ancient species into modern categories. The fact that ancient species blur the distinctions between modern day taxonomic groups (such as arthropods and onychophorans) shows that what we recognize as large taxonomic groups (such as what we call “phyla”) are in fact best described as monophyletic groups in nested sets.
The fossil record supports the gradual acquisition of the characteristics we see in modern groups and subsequently use for classification.
The key defining features of arthropods (specialized segmentation, specialized appendages, hard exoskeletons, compound eyes, and a host of other characteristics) do not appear all at once in any one species in the fossil record. To say that “arthropods” arise and diversify in the Cambrian is not to say that fully-fledged (i.e. crown-group) arthropods appear suddenly out of nowhere. Rather, we see a range of species that demonstrate that the defining characteristics of crown-group arthropods were acquired over a long period of time. While stem-group species are not likely to be direct ancestors of crown-group species, their presence in the fossil record (and the nested hierarchy their characteristics produce) provides a means to determine the order in which the defining characteristics were assembled.
The Cambrian, though “explosive,” should not be misunderstood to be instantaneously producing crown-group species.
There is no doubt that the Cambrian “explosion”, was a spectacular diversification event – but it did not produce species with fully modern character sets instantaneously. Rather, we see a diversification of stem-group organisms, the sequential and serial addition of characteristics in some lineages over time (with the loss of other lineages), and the eventual production of character sets in successful monophyletic groups that we retrospectively recognize as taxonomic groupings.
In the next post in this series, we’ll examine the Cambrian origins of the lineage that eventually led to humans – the vertebrates.