Devonian

Published on June 29th, 2018 | by David Marshall

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The Expedition

Welcome to this special series of podcasts relating to a fieldtrip that I have been invited on by Dr Martin Brazeau of Imperial College London.

I’m being flown out as the Palaeozoic arthropod “expert” of the team and I’ll be there to deal with all the eurypterids and phyllocaridids we come across, along as documenting the whole process for outreach and hopefully your enjoyment.

In all, this trip will last around 6 weeks, during which time we’ll have no internet, electricity, running water or even any toilets. It’s going to be a gruelling trip, but hopefully one that will give you an insight into what life is like in the field. You will join us as we discuss the science, prepare for the trip, arrive in the field, go out digging and finally wrap things up. You will experience all the highs of discovering new and exciting fossils and the lows of when we’ve just all had enough. This expedition is a unique opportunity to share with you a single research project from start to finish, rather than just the results.

Context

In this first episode, we contextualise why we’re going into the field. What is the current lay of the research landscape? What we already know? and what are we aiming to find out about the early evolution of the jawed vertebrates, a group to which we ourselves belong?

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The vertebrate evolutionary tree. (a) the origin of jawed vertebrates, (b) the origin of crown-group gnathostomes (i.e. the common ancestor of all living jawed vertebrates + all descendants). Ostracoderms are a grade of jawless vertebrates covered in boney armour. Placoderms are the earliest jawed vertebrates and are also possess boney armour. They are currently thought to comprise an evolutionary grade, rather than a true group . Image from Qu et al. 2013, Scales and Dermal Skeletal Histology of an Early Bony Fish Psarolepis romeri and Their Bearing on the Evolution of Rhombic Scales and Hard Tissues. PLoS ONE 8(4) CC BY 2.5.

 


Hagfish are a group of living jawless vertebrates with the disgusting ability to secrete a microfibrous slime, which expands on contact with water. One of the uses of this slime is to clog potential predators gills.

Lampreys are another group of living jawless vertebrate. Their mouth has evolved into a sucker-like disc lined with horny ‘teeth’ made of the structural protein keratin. Some species of lamprey are parasitic. They attach to unsuspecting fish using their sucker-like mouth and rasp at the flesh using a horny tongue. Image from Wikimedia Commons CC BY-SA 3.0

 

Recent advances in the study of early jawed vertebrates from the Silurian and Early Devonian have changed perspectives on the evolution of the bony exoskeleton. Janusisicus from the Early Devonian of Siberia revealed a combination of features form bony fishes and placoderms, corroborating a hypothesis that the scaly exoskeleton of sharks and their relatives is an ‘advanced’ feature, rather than an ancestral state of jawed vertebrates. Image: Giles et. al. 2015.

 

Placoderms came in all shapes and sizes. One of the largest was Dunkleosteus, which lived during the Late Devonian period, about 358–382 million years ago. The largest species, D. terrelli, is estimated to have reached 6 meters in length and approximately 1 tonne in weight. Dunkleosteus belongs to a group of placoderms called arthrodires, the oldest of which have been found in the Silurian of China. Image from Wikimedia Commons, Public Domain.

The placoderm Bothriolepis was one of the most abundant early vertebrates during the Middle to Late Devonian. It belongs to a group of placoderms called the antiarchs. The oldest antiarchs so far discovered have been found in Silurian-aged sites in China.

 

Acanthodians, also known as ‘spiny sharks’, are a group of early jawed vertebrates known from the Silurian to Devonian periods, approximately 440 – 360 million years ago. Their evolutionary relationships have been the subject of much debate. Historically, it was though they represent a true group that diversified close to the origin of jawed vertebrates. It has also been suggested that some acanthodians might be more closely related to the boney fishes. More recently, it has been suggested that acanthodians represent a grade more closely related to living sharks and rays. Pictured is Obtusacanthus, an acanthodian from the earliest Devonian of Canada.

As well as looking for jawed vertebrates we are also on the hunt for ostracoderms, which are boney, jawless fishes. As their name suggests they lacked the classic vertebrate feature of a jaw and many other diagnostic characteristics, such as paired fins and a boney internal skeleton. Instead, many of the different ostracoderm groups had a boney ‘dermoskeleton’ which encased their bodies from the outside. They are really important for understanding the assembly of the early vertebrate skeleton and were the dominant fishes in early vertebrate faunas (Silurian to early Devonian).

NMC.30417 Homalaspidella is a cyathaspidid heterostracan from the Pridoli (420 million years ago in the Silurian) is an example of a jawless fossil fish we may expect to find in the Canadian Arctic (housed in the Canadian Museum of Nature, Ottawa).

NMC.19713 Vernonaspis is another cyathaspidid heterostracan from the Pridoli (Silurian 420 million years ago) is also an example of a fossil we may expect to find in the Canadian Arctic (housed in the Canadian Museum of Nature, Ottawa).

 

The earliest jawed vertebrates were contemporaneous with the ostracoderms (armoured jawless fishes). There is some evidence that jawed vertebrates preyed upon their jawless cousins, however the community dynamics of early vertebrate faunas are largely unknown. Martin, Dave and the team hope that new discoveries from Cornwallis Island will help us understand how the evolution of the gnathostomes effected early vertebrate communities. Image from Choo et al. 2014, The largest Silurian vertebrate and its palaeoecological implicationsScientific Reports 4. CC 4.0

Logistics

We now move on to discussing the logistics of the trip. How do you go about making this kind of expedition happen? What are some of the challenges we will face? What will life be like in the camp? and how will we get our priceless fossils home?

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We first arrived in Ottawa a few days before headed to Cornwallis Island to make our final preparations. We still needed to get the last bits of equipment, practice safety procedures and squeeze in a visit to the Canadian Museum of Nature to check out their fossil collections.

The single most important thing was to bring along enough chocolate. Dave is here helping Martin’s mom prepare $120 worth of almonds in chocolate. Having such calorie rich food is important because we will be trekking many kilometres in the daytime, breaking rocks and carrying heavy loads, all in cold temperatures.

Another important thing we are taking are maps, we therefore stopped in at the World of Maps to get ours printed. We will be needing these, not only to find our way around the island, but to record the position of the discoveries and also the nature of the rocks we encounter. Geological mapping 101.

Here Anthony points out Cornwallis island on the globe. It’s pretty far north…

Another important thing, from a safety perspective, is to get familiar with the weapons we will be taking into the field. The team therefore headed to the shooting range to get hands-on experience of the rifles, ammunition and reloading process.

Since the condition of a gun changes with every round shot, we had to calibrate each rifle so that we can be sure of hitting what we’re aiming at. Our lives may depend on this if we encounter an aggressive polar bear.

Initially, our shots were a little off…

But soon we managed to get these to a good level of accuracy over 90m.

Here, Martin, Emma and Anthony look through drawers of Canadian Arctic fossils at the Canadian Museum of Nature’s Collections in Gatineau, Quebec.

It was important for us to visit the collections held in the Canadian Museum of Nature to get our eye in for the types of fossils we may be finding.

A jaw from a small Acanthodian jaw: a spiny shark from the Canadian Arctic, Canadian Museum of Nature

A frontal appendage of a carcinosomatid eurypterid. Each segment of this leg (top margin) possesses large sickle-shaped spines that were used for prey capture. Eurypterids, such as this, would have been amongst the top predators in the Silurian, but in the Devonian, the ‘age of the fishes’, they took a big hit to their diversity.

A little on our home for the next few weeks: Cornwallis Island.

The field area showing the kind of environment we’ll be working in. These rocks were formed in shallow coastal waters during the Late Silurian, recording barrier reefs and muddy carbonate lagoons. They’re made up of limestone muds with corals and sponges providing a framework, similar to what is seen in the modern day reefs. Image: Jeff Packard

The upside to the conditions is that there will be huge amounts of exposure and access to rocks is guaranteed. On the downside, these rocks will likely be weathered, so there will be plenty of splitting required to get fresh surfaces. Image: Jeff Packard

Access into the field is only feasible by air, given the amount of equipment we are to be carrying. Aircraft capable of landing in the field are helicopters and the Twin Otter plane, the latter of which can be seen here flying over our field area.

Check back here in a couple months to see what life was like in the camp and to see the amazing fossils that we’ve (hopefully) found!

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