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 examine stem groups on the chordate lineage to better understand how the vertebrate body plan was assembled.
In our last post in this series, we introduced the defining features of chordates:
- a hollow dorsal nerve cord
- a rod-like, flexible structure called the notochord
- a pharynx (with pharyngeal openings, or “gill slits”)
- a tail that extends past the anal opening (a “post-anal tail”)
We further noted that vertebrates are chordates – since they have all of the above features – but also have a brain encased within a skull, and a backbone. Non-vertebrate chordates include two groups with present-day representatives – cephalochordates (lancelets) and tunicates (sea squirts). Together, these three monophyletic groups form the crown group chordates (i.e. all living chordates, their last common ancestral population, and all of its descendant species, whether living or not). Crown group vertebrates, in turn, are nested within chordates, and chordates themselves are nested within a larger group known as deuterostomes:
Phylogenetic relationships within deuterostomes, showing the nested relationships of vertebrates (blue box) within chordates (red box), and chordates within deuterostomes (green box). All vertebrates are thus chordates, but not all chordates are vertebrates, and so on. The arrow indicates that the position of tunicates and cephalochordates may in fact be reversed. Not to scale.
Building the vertebrate body plan: stem-group deuterostomes
Having located vertebrates in their broader phylogenetic context, we can now trace some of the key innovations that ultimately led to the vertebrate body plan. As we discussed in the last post, this tracing process is not based on finding direct ancestors of crown-group vertebrates, but rather by looking in the fossil record for organisms that branch off the vertebrate lineage at various time points, carrying with them their up-to-that-point-in-time set of characteristics. In other words, we can infer a great deal about the true vertebrate lineage by examining stem-group species in the fossil record.
One challenge for reconstructing the vertebrate family tree is the fact that stem-group deuterostomes (and for that matter, stem-group chordates) were soft-bodied animals, and thus did not fossilize readily. Indeed, what we know about these ancient, soft-bodied species comes from a handful of key fossil sites, where fortuitous circumstances led to their preservation – sites such as the Burgess Shale in Canada, and the Chengjiang region of China. At these sites, we see brief geological “snapshots” of life in the Cambrian period. The Chengjiang fossils capture a window at 525-520 million years ago, and the Burgess Shale captures a slightly later period of the Cambrian at 505 million years ago. Moreover, the flourishing life we see at these sites (and times) of exceptional preservation indicates that the “regular” fossil record of this period, where circumstances did not favor the preservation of soft-bodied organisms, is woefully sparse.
One group of organisms that sheds light on the early origins of deuterostomes is the vetulicolians – a group of Cambrian organisms that remained somewhat of a mystery until recent work, based on new fossils found in Chengjiang, places them as a likely stem-group deuterostome lineage.
The evidence for Vetulicolians as stem-group deuterostomes is based on what appear to be a pharynx and pharyngeal openings. These structures presumably allowed them to expel swallowed seawater and feed as suspension feeders. The fact that a group of species has these traits – without other defining characteristics seen in crown-group deuterostomes – strongly suggests that an early step towards the “vertebrate body plan” was the development of pharyngeal openings:
Building the vertebrate body plan: stem-group chordates
The Cambrian also shows evidence of stem-group chordates in addition to stem-group deuterostomes. One such organism, known from the Burgess Shale, is Pikaia gracilens – a species immediately noted for its resemblance to cephalochordates upon its discovery in the early 1900s. A recent large-scale re-evaluation of Pikaia places it as a stem-group chordate. In support of this placement, Pikaia exhibits a pharynx with pharyngeal openings, a notochord, and a dorsal hollow nerve cord – but appears to lack other defining chordate features (such as a post-anal tail). Accordingly, this evidence supports the hypothesis that the next “step” towards crown-group vertebrates was the development of the dorsal nerve cord and notochord:
Accordingly, the next characteristic to be a acquired would be a post-anal tail, to finally arrive at the defining features of what we now recognize as a hugely successful monophyletic group: the crown-group chordates.
Building the vertebrate body plan: Cambrian crown-group vertebrates
The Chengjiang area is perhaps most famous for the discovery of Cambrian species that are thought to be true vertebrates, including Myllokunmingia and Haikouichthys. These species are thought to be the earliest-known examples of jawless fish, with skulls and backbones made of cartilage. If indeed these species are true vertebrates, they would be part of the vertebrate crown group (though as we shall see in later posts, these species also sit as a stem group to later sub-categories of vertebrates as additional characteristics, such as jaws, are added):
Summing up – from deuterostome to vertebrate
Taken together, what we have seen is that though the vertebrate body plan first appears in the fossil record in the Cambrian period (due to its fortuitous preservation at Chengjiang), there are numerous species in the Cambrian that are identifiable as stem groups on the vertebrate lineage. These stem groups show us that the vertebrate body plan was assembled over time in a stepwise fashion, and that its “sudden appearance” in the Cambrian record is in fact not sudden at all, but rather the end result of a process that extends much deeper into the past.
In the next posts in this series, we’ll trace the vertebrate lineage forward towards more recent times, and examine the wonderful diversity of forms present in the vertebrate crown group.