Two new theropods in November!

November's not quite done yet, and we've seen two important new theropod dinosaurs named this month alone. Both of these are scientifically significant, and both come from Utah, but they lived at different times and are from different groups. Here, I will briefly introduce you to these two new dinosaurs, Lythronax and Siats.

Siats meekororum
First up is Siats meekororum, from the Late Cretaceous [1]. It lived 98 million years ago, which makes it the youngest (geologically) known allosauroid (like Allosaurus) from North America. It's also the first time a dinosaur from the theropod group known as the neovenatorids has ever been found on this continent. Siats was a top predator, living long before tyrannosaurs, and may have been as much as 12m long. It is known from a single incomplete individual, that was skeletally immature, and about 9m long, suggesting a fully grown individual would have been longer. Siats helps to fill a 25 million year gap between larger North American predators like Acrocanthrosaurus and Tyrannosaurus, and palaeontologists were trying to understand why no large predator fossils had been found in between. They also found evidence of small bodied tyrannosauroids living alongside Siats, the first time these two groups have been found living together. We now know that there were large predatory theropods throughout the Cretaceous, and the presence of these other large predators likely prevented early success of they tyrannosaurs until after these allosauroids had disappeared. This was an important find by extending the evolutionary range of allosauroids, furthering our understanding of tyrannosaur evolution, and revealing a new coexistence between two groups of predators.

For a picture and another post on Siats, see Brian Switek's post here.

Lythronax argestes
The second big theropod news of November was Lythronax argestes, meaning 'southern king of gore' [2]. Lythronax is the oldest known tyrannosaurid known from a single adult individual with a partial skull and bits and pieces of the rest of the body. It would have been about 8m in length, and lived during the Late Cretaceous, about 80 million years ago. As there is a partial skull known, we know a bit about it's behaviour. Lythronax had a similar skull shape to Tyrannosaurus, with a short and narrow snout, wide back of skull, and forward-facing eyes allowing for depth perception, which is generally considered to be a characteristic of predators. This skull shape was thought to be a derived characteristic, but this find pushes the appearance of this skull shape back by several million years. It had large serrated teeth that were used for slicing through flesh, but also capable of crushing bones with its well built skull and jaw. Currently, Lythronax represents the largest predator from the ecosystem it lived in, which was cut off from the rest of North America from the Western Interior Seaway. This find has changed some of our understanding of tyrannosaur evolution as it has features that were not previously found so early, which has changed how we thought certain tyrannosaurs were related. Lythronax will be a very important find in the understanding of tyrannosaur evolution. 

For more details, check out Dave Hone's blog post in The Guardian about Lythronax, or download the paper itself, which is free and open access!

Pictures and skull reconstruction of Lythronax argestes from Loewen et al [2]

I hope you've enjoyed this short introduction to two important new theropod finds! These will both be very important in understanding theropod evolution in the future. 

References
1. Zanno and Makovicky. 2013. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nature Communications 4.
2. Loewen et al. 2013. Tyrant dinosaur evolution tracks the rise and fall of Late Cretaceous oceans. PLoS ONE 8: e79420. --> Freely downloadable here doi:10.1371/journal.pone.0079420

Pleistocene Megafauna

In the past, we've been focusing on very old fossils, mainly from the Mesozoic. This week, we're going to talk about some fossils that are a bit younger, specifically, from the Ice Age. These animals are commonly referred to as the Pleistocene Megafauna, where the Pleistocene is the time period (epoch) that the most recent ice age occurred and these animals lived in (2.5 million to 11 700 years ago), and megafauna refers to the large size of these animals compared to what we see today. 

A number of these fossils are very well known, like mammoths, and sabre-toothed cats, but other ones are less well known. Although Albertan palaeontology tends to mainly be dinosaurs and similarly aged animals, there are actually a number of the Pleistocene megafauna fossils that can be found in Alberta. Relatives of modern animals like horses (from the genus Equus, the same genus as modern horses) and Bison are often found in Alberta, as well as mammoth fossils. Mammoth fossils can be found near Medicine Hat, as well as several places in western and northern Canada. Mammoths are extinct relatives of elephants that lived in colder habitats during the Ice Age. They are well known for often having long, curved tusks, which are found commonly in Canada. As mammoths lived in glacial environments, they can often are found mummified, so we know a lot about their biology.

Mammoth skeleton. Photo by WolfmanSF

Another example of a Pleistocene megafauna that lived in Alberta during the Ice Age was Jefferson's ground sloth, Megalonyx jeffersoni. This giant ground sloth measured 3 m long, and as much as 1 ton in weight. Shockingly, this is only medium sized among the giant ground sloths, but it's still significantly bigger than any sloths alive today. Its fossils are more rare than mammoths, but they can be found in Alberta, Saskatchewan, the Yukon, and NWT. Its peg-like teeth and plantigrade (flat) feet allowed the animal to reach up on its hindlimbs and pull leaves down from trees and break down the tough plant material. They also had long claws on the forelimbs probably used for stripping branches. 

Megalonyx skeleton. Photo by Daderot

Finally, who knew there were camels in Alberta? One species of camel, known as Yesterday's Camel, has also been found in Alberta.  It was over 2 m tall at the shoulders, and weighed 800 kg, and is thought to have been an opportunistic herbivore, meaning it would eat whatever plants it could find. The camelids, horses, and mastodons all went extinct in North America at approximately the same time as Clovis tools (early human tools) became common. This has lead to the belief that increased human activity in North America actually drove these animals to extinction as we started to hunt. This is also possibly the case for the disappearance of the Pleistocene megafauna from around the world.

There are many other megafauna found in Alberta, and around the world, but here was a brief overview and some examples from Alberta and western Canada. I hope you found this brief report interesting!

References
Jass, C.N., et al. 2011. Description of fossil muskoxen and relative abundance of Pleistocene megafauna in central Alberta. Canadian Journal of Earth Science 48: 793-800.

Marine Reptiles in Alberta

You may have noticed that there was no blog post last week. Sorry about that! That is because the writer of this blog (me - Liz) was busy last week starting a PhD at the University of Southampton in the United Kingdom. I was so busy last week I just didn't have a chance to write! But now we're back!

Although we've talked in passing about extinct marine reptiles in the past, it has never been in detail, which is why I decided that this week we would talk a bit about the sea creatures often found in Alberta. In order to talk about the marine reptile fossils found in Alberta, I think it's important to explain some of geological history of the region.

In the mid to Late Cretaceous, a large water body called the Western Interior Seaway ran through North America, connecting the Arctic Ocean to the Gulf of Mexico. This large seaway lasted for about 100 million years, and caused nearly the entire province of Alberta to be underwater. This large seaway allowed for many large and small marine animals to invade, including sharks, fish, and of course, large marine reptiles. 

Image of modern North America in the mid-Late Cretaceous, covered in seaways

Although there are many groups of marine animal fossils that have been found in Alberta, we are going to focus on the two groups of marine reptiles that have appeared. The first group is the plesiosaurs. Plesiosauroids are the long-necked marine reptiles in the group Plesiosauria. They differ from their close relatives, the pliosauroids, by (generally) having very long necks and small heads, while pliosauroids (generally) have short necks and large heads. As of 2012, nearly 30 plesiosaurid specimens had been found in western Canada, with just over half of them coming from Alberta [1]. Although not all are identifiable to genus or species, most of them are either from the family Elasmosauridae, which includes the plesiosauroids with the longest necks of all, or the Polycotylidae, which have shorter necks and closely resemble pliosaurs. Elasmosaurids from Alberta include Albertonectes (found near Lethbridge) and Wapuskanectes which was found in a Syncrude mine near Fort McMurray. Because the oil sands are marine deposits, it's quite common for marine fossils to be found in these mines. Trinacromerum (from Dinosaur Provincial Park) and Nichollsaura (also from near Fort McMurray) are some of the polycotylids found in Alberta. Plesiosaurs are also found in Saskatchewan, Manitoba, and occasionally in BC.

Artists impression of Albertonectes by Smokeybjb

Trinacromerum by Nobu Tamura

The second group of marine reptiles found in Alberta is the mosasaurs. Mosasaurs lived only in the Late Cretaceous, and were the dominant marine predators during this time. They were strong swimmers, and even better predators, reaching lengths from 3-18 m and having double-hinged jaws much like modern snakes, allowing them to open their mouths wide and swallow prey. Prognathodon, a rare, large-jawed mosasaur has been identified by a few specimens in Alberta, including one nearly complete specimen from near Lethbridge [2]. These were found by the Korite International, an ammolite mining company, while mining for ammolite. Another possible specimen was discovered in 2012 in a mine as well. Another mosasaur, Plioplatecarpus, which has proportionally larger eyes than other mosasaurs, was identified from an incomplete specimen found in southern Alberta.

Artists impression of Prognathodon

Plesiosaurs and mosasaurs are just two of the groups of marine fossil animals that have been found so far in Alberta. Other groups include fishes, sharks, and possibly pliosaurs as well. I hope this has given an insight into the non-terrestrial fossils that can be found in Alberta, and the kinds of marine reptiles often found!

References:
1. Kubo, T., et al. 2012. Albertonectes vanderveldei, a new elasmosaur (Reptilia, Sauropterygia) from the Upper Cretaceous of Alberta. Journal of Vertebrate Paleontology 32: 557-572.
2. Konishi, T., et al. 2011. New exceptional specimens of Prognathodon overtoni (Squamata, Mosasauridae) from the Upper Campanian of Alberta, Canada, and the systematics and ecology of the genus. Journal of Vertebrate Paleontology 31: 1026-1046.

Dinosaurian Myths: Take 2

Last week, we introduced you to a few different dinosaur-related myths ("Brontosaurus, all dinos were big, and Triceratops is no more) that are particularly irksome in my opinion. This week, we're going to continue on this theme with three more common dinosaurian myths! Since last week was 1-3, this week will continue with 4-6.

4. All large, prehistoric reptiles were dinosaurs
This is one that I've touched on before at the very beginning of Mesozoic Mondays, when I explained, in detail, exactly what a dinosaur is, and for that matter, what isn't. Dinosaurs are a group of organisms united by a very specific suite of characters, not just a group of big, scaly creatures. In fact, all of those swimming, and flying things normally called dinosaurs in a kid's toy set or book are not even dinosaurs! The large, aquatic reptiles come from several distinct groups, not closely related to dinosaurs. These include ichthyosaurs, mosasaurs, plesiosaurs, pliosaurs, and many more. "What about those scaly flying creatures like a pterodactyl?" Nope, not dinosaurs either. These are pterosaurs, which are closely related to dinosaurs, but not actually in the Dinosauria. "Does that mean that not a single dinosaur swam or flew?" Absolutely not. Dinosaurs would have swam occasionally to get across water, and there is evidence some spent more time in the water than others. However, there are no fully aquatic dinos that evolved paddle-like limbs to swim around. These are just other prehistoric, and indeed Mesozoic reptiles. When it comes to flying, there are of course a whole suite of dinosaurs that flew: they are the dinos that evolved into birds, and in fact birds. "Ok fine, but what about those guys with the sails on their back that we always see in books? Those are dinosaurs, right?" you may be asking. And the answer is: nope, they lived even before the dinosaurs, and are ancestors to modern mammals. In fact, they are more closely related to you and I than to dinosaurs! So spread the word: not everything was a dino! 

5. Mammals outcompeted/were more fit than dinosaurs
This common myth comes in several different forms, but the general thought is that mammals were somehow more fit/better than dinosaurs  and therefore outcompeted them and that's why mammals survived and dinosaurs didn't. This is, of course, false on many different levels. First of all, while mammals were living at the same time as dinosaurs, they were substantially smaller (in general), and were not (also in general) capable of competing with dinosaurs for anything. Dinosaurs were widely successful, living in every ecological niche on land, while mammals were mainly filling the niche of small, scavenger-like creatures. When the meteor hit the Earth at the end of the Cretaceous, the wide scale environmental changes that ensued made it very hard on the dinosaurs, which in general required a large amount of food to survive. It was then that the mammals were able to truly expand into the niches left empty as the dinosaurs died, and then that mammals evolved into the diverse group they are today. While this myth is mainly false, there is a slight ring of truth to it in that mammals were better able to adapt to the changes once the meteor hit, than the larger dinosaurs. However, that does not mean that they were better in general, and they were not responsible for the demise of the dinos.

6. Stegosaurus (or other large dinosaurs) had a secondary brain in its hips.
Still, to this day, this boggles my mind whenever I hear it. In the 19th century, American palaeontologist Othniel Charles Marsh noticed that large dinosaurs often have big cavities in their hips. As this is associated with the area that the spinal cord passes through, he suggested that this cavity actually housed a second brain. Unfortunately, it's difficult to know exactly what this was for. Birds often do show a slight expansion of the spinal cord in this region to help regulate their limb movements, but this is hardly on the scale of a second brain. It's likely that this area actually housed something called a glycogen body, which is also seen in the hips of birds, and helps to store energy. 

Stegosaurus. Image by Nobu Tamura

I hope you enjoyed and learned a bit more about these common dinosaurian myths! If there's anything that you often wonder about in palaeontology, let me know, and I may discuss it in my next blog!

A Few Dinosaur Myths

This week, I'm going to tackle a few dinosaur-related myths that I find particularly irksome, or I think are important for people to understand are not true. So read-on to learn about some commonly discussed dinosaurian misconceptions!

1. All things "Brontosaurus"
This is less of a big thing now than it was throughout my childhood, as it seems that kids books are finally catching up with the science, but it is still around. Here's the deal: Brontosaurus does not exist. I know, you may need to take a seat, as many of you may have had a favourite dinosaur as a kid called Bronty. There's little that bugs me more than picking up a kids book or toy, looking at it, and discovering it's full of Brontosaurus labels. So whatever happened to Brontosaurus? Well if you've been following Mesozoic Mondays for a while, you'll remember that there are often problems with naming new species, and this is one of those instances. 

In 1877, well known American palaeontologist Othniel Charles Marsh named a new species, Apatosaurus ajax, from an incomplete, juvenile skeleton. A few years later, Marsh named a new, much larger (and more complete) skeleton, "Brontosaurus excelsus", mistakingly believing that it represented a new species. This name was cemented in the public's mind (the thunder lizard!), as it was the largest dinosaur ever discovered at that time. Unfortunately, as early as 1903, it had been suggested that these two species were actually the same, and as Apatosaurus was named first, it has priority, meaning every Brontosaurus you've actually read should be Apatosaurus. While scientists have been using Apatosaurus for more than 100 years, it's taken much longer to make it to the public, so spread the word! Tell your kids! No more Brontosaurus please!

Apatosaurus from Jurassic Forest

2. All dinosaurs were big
I'm not exactly sure where this idea comes from, but there seems to be a belief that all dinosaurs were these massive animals bigger than any animals alive today. While many of them were big, there were also just as many small dinosaurs, similar in size to modern animals. In general, an ecosystem wouldn't work if every animal was huge. You need little animals too to make the world go 'round!  In fact, part of why dinosaurs were so successful was that they were able to occupy many different niches with their wide range of sizes. Early dinosaurs like Eoraptor were small, weighing in at only 10 kg, while many derived, later dinosaurs were also small. And there were small dinosaurs in many groups too. There was Stegoceras, a 10 kg dome-headed (pachycephalosaurid) dinosaur from the Cretaceous of North America, or Anchiornis, a tiny pigeon-sized theropod from the Jurassic of China, and even Psittacosaurus, an early relative of the horned dinosaurs (ceratopsians) was only 20 kg. Not all dinos were the gigantic fearful animals from the movies!

Image showing the size of Anchiornis. Image by Serenthia

3. Triceratops is no more
This is something that came about the summer Jurassic Forest opened, and I remember very well how many people were asking us about this on the trails. Now this is more of a recent misconception, and probably is not really a myth as it's only been around for a few years, but it is a common misconception brought on by the media that really drives me crazy. This idea stems from some recent work by Jack Horner's lab, regarding the horned dinosaurs Triceratops and Torosaurus. The basic gist of it is that he believes that what we currently consider to be Triceratops is actually a sub-adult (therefore not fully grown) individual, and that the slightly larger Torosaurus actually represents the adult of this species. Most news stories picked this up by suggesting that Triceratops is no more... which is not correct. First of all, if this hypothesis is correct, they would still both be called Triceratops, only what we currently view as an adult Triceratops would not be the adult, but a subadult. They still look pretty similar, and it would still be Triceratops, but bigger! And the second, and very important thing to note, is that this may not be true. Not all researchers agree with this idea, and believe that Triceratops and Torosaurus are still distinct species. 

So, next time someone says "Triceratops doesn't exist", you know what to say!

Skulls of Triceratops and Torosaurus from Longrich and Field (2012)

So I hope you enjoyed (and more importantly, learned something!) this little introduction to dinosaur-related myths. Next week, we might continue on this topic with a few more common misconceptions! Stay tuned!

References:
Longrich NR, Field DJ (2012) Torosaurus Is Not Triceratops: Ontogeny in Chasmosaurine Ceratopsids as a Case Study in Dinosaur Taxonomy. PLoS ONE 7(2): e32623. doi:10.1371/journal.pone.0032623

Taphonomy

This week, we're going to talk about something that is arguably one of the most important things to understand in palaeontology: taphonomy. A basic definition of taphonomy is the study of everything that happens to an organism after it dies, and before we find the fossil millions of years later. 

Understanding taphonomy can give us many clues about the environment that an animal lived and died in. In particular, it can be very useful when it comes to studying bonebeds, geological deposits that are full of many bones. Here, I'll go through a step by step example of the things you can learn from bonebed taphonomy from the moment you find the bonebed, to the fine details you can find in a lab. 

When you first find a bonebed, there are several features that you notice first, like the number of bones, the number of taxa, how the bones are orientated, and how complete the skeletons are, which give us a wide range of information. The number of bones can obviously tell you how many animals are found there, while the number of taxa can tell you about how those animals came to be there. For example, bonebeds that consist of mainly one species, and all ranges of sizes are likely to be from a herd or pack of animals that all perished at one time in a catastrophic event. On the other hand, bonebeds with several taxa may be characteristic of something like a predator trap where many animals go (like a watering hole), and are then killed by predators, leaving several different kinds of bones over a long period of time. These are termed monotaxic (only one taxa present), multitaxic monodominant (more than one taxa, but there is primarily one), or multitaxic multidominant (more than one taxa, with more than one dominant taxa). 

The Edmontosaurus bonebed in Edmonton. An example of a multitaxic monodominant bonebed. Photo by Liz Martin.

Next, you might notice how the bones are orientated. Some bonebeds will show an obvious  orientation to the bones, which can tell us that the bones were transported by something like a river, that will orient the bones in the direction of the flow. By studying and mapping these bones, we can understand palaeoflow and something about the ancient rivers. Completeness and degree of articulation (whether the bones are joined together in the way that a skeleton normally is or jumbled up and separated) can tell us more about how those bones came to be in that place. A complete, articulated skeleton tells us that an animal died and was undisturbed after it died. Single, isolated bones mean that the skeleton was disturbed either during death (during an attack), or after death. Bonebeds full of disarticulated bones can tell us more about transport (as an animal carcass is transported by a river, it begins to come apart), or scavenging before fossilisation. 

Example of a map of a ceratopsian bonebed showing the orientation of different bones [1].

Finally, once all the bones have been mapped and palaeontologists have returned to the lab to look at the bones, they study another important aspect of taphonomy: the fine structure of the bone. Looking at the fine detail of the bone can reveal marks like insect borings, scratch marks, and bite marks, which can tell us more about how the animals came to be in that position. A devastating bite mark may show us how it died, or bites can show that the carcass was scavenged after it died, and in some cases, by what kind of animal. Scratches can show trample marks, or possible predatory marks with an animal scratching at a carcass to pull off meat. Insect borings can tell us about decomposers that may have infested the carcass after it died. All of these different pieces of information can be put together to understand more about how the animal(s) died, the environment that it lived in and much, much more. 

Possible beetle feeding traces on a Jurassic dinosaur Camptosaurus from the US [2].

Bite marks on a humerus of Saurolophus [3].

This has been a quick review of different aspects of taphonomy, and I hope you have learned a bit about what it can teach us. In the future, we'll talk more in detail about different aspects of taphonomy. Stay tuned!

References
1. Eberth, D. A., et al. 2007. A practical approach to the study of bonebeds. In Rogers, R. R. et al. (eds.) Bonebeds: Genetic Analysis, and Paleobiological Significance. pp 265-333.
2. Britt, B. B., et al. 2008. A suite of dermestid beetle traces on dinosaur bone from the Upper Jurassic Morisson Formation, Wyoming, USA. Ichnos 15: 59-71.
3. Hone, D. W. E., and Watabe, M. 2010. New information on scavenging and selective feeding behaviour of tyrannosaurids. Acta Palaeontologica Polonica 55: 627-634.
Background info:
Eberth, D. A., et al. 2007. A bonebeds database: classification, biases, and patterns of occurrence. In Rogers, R. R. et al. (eds.) Bonebeds: Genetic Analysis, and Paleobiological Significance. pp. 103-221.

Cladistics - How we understand relationships of animals

After writing this blog for a year, I've come across a number of terms that I've had to define as many non-biology people may not understand them. Then I thought maybe it would be a good idea to give a quick review of some of these and how they relate to palaeontology. So here goes...

The first important thing to understand is where modern taxonomy or classification of plants and animals comes from. The first important word is taxon. A taxon is a single group of animals. This group can be any group from species to phylum, depending on what is being talked about (for a review on the modern classification system of animals, check out our post 'Where do dinosaur names come from?'). So as an example, lets use Tyrannosaurus rex. It's classification is as follows:

Tyrannosaurus rex

Kingdom Animalia

Phylum Chordata

Class Reptilia

Order Dinosauria (this isn't completely accurate, but we will use it here as an example)

Family Tyrannosauridae

Genus Tyrannosaurus

Species Tyrannosaurus rex

In this case, taxon can refer to the species, genus, family, order, etc., as long as you are only referring to that one group. There are even subgroups, like subfamily, tribe, etc. and are also referred to as a taxon. If someone wanted to refer to a number of groups at once, we use the term taxa. That could be a number of genera (plural of genus) within a family, or something like that. 

The next important thing to talk about is cladistics, the main point of this post. Cladistics is a way of grouping organisms based on how many unique characteristics they share with each other. These characteristics must not be the ancestral state, or plesiomorphic, which means the character is retained from its ancestor. For example, most tetrapods have some type of front limb (like an arm or paddle), which is because fish have fins, and tetrapods evolved from fish. This character would therefore not be used to assess how closely related a tetrapod group is to another. In contrast, apomorphies, or derived characteristics are used to group closely related or separate distantly related organisms. To use the limb example again, snakes have lost their limbs, which is considered to be a derived characteristic. The loss of limbs could, therefore, be used in cladistics. 

As implied above, each character is coded into a character state using 0 as primitive, and 1-4 as derived. For example, using limbs again, an animal with front limbs would be coded as 0, as that is the basal state, whereas a snake would be coded as 1 for the derived state. This can become much more complicated where you could also have something like a state for a paddle-like limb for a swimming animal, and more. For a group of organisms and characters, you create a character matrix full of numbers (0-4 usually), which is then uploaded into a computer program like PAUP or TNT. Using a complex number of algorithms, these programs work out how closely or distantly related taxa are, and spit out a cladogram. Think of a cladogram as a kind of family tree, where the relationships between groups can be seen. 

An example of a character matrix for ceratopsian dinosaurs from Ryan et al. [1]. Each taxon has the same number of characters with states from 0-2. ? denotes where the state is unknown due to the lack of preservation, which is often the case in fossils.

An example of a cladogram made form the matrix above from Ryan et al. [1]. 

If the relationship is well supported and can be distinguished, there will be a branch of just 2 branches on each side. For example, you can see on the bottom that Chasmosaurus and Pentaceratops are split into two, suggesting these are very closely related, and well supported. In contrast, at the base of the "tree", you'll see that Protoceratops, Turanoceratops, Zuniceratops, and the large branch containing everything else are all coming off of the same line, meaning there isn't enough evidence to fully understand the relationship between those groups. A cladogram is made of clades, which can be any group of organisms that includes all descendants of a particular ancestor. For example, a clade may be the entire tree above, or could be the group in the middle that includes Centrosaurus, Coronosaurus, Styracosaurus and Spinops, or the small clade at the bottom with Chasmosaurus and Pentaceratops, and many many more. 

Another example of a cladogram for ceratopsians from Farke et al. [2]

Above is another representation of a cladogram, which may be more easy to understand. When comparing this one with the one from Ryan et al. [1]  you can see some differences. First of all, Protoceratops is no longer included in that group that was unresolved in the first one, however, Zuniceratops, Turanoceratops, and everything else is still not clear. Chasmosaurus and Pentaceratops still make up a group (seen towards the top) while Avaceratops, Albertaceratops, and the rest are now unresolved. 

The final term that I think is useful to talk about is 'sister groups'. When something is considered to be a sister group to something else, it means that these are closer to each other than anything else. It also means that these groups can swap places on the cladogram and it would not change the meaning. Looking at the above cladogram, Pachyrhinosaurus canadensis is sister to Pachyrhinosaurus lakustai. The group that includes both Pachyrhinosaurus species is in turn sister to Achelosaurus, and the group that includes all three is in turn sister to Einiosaurus, which makes that group sister to Rubeosaurus. That whole group is then sister to the group that includes Spinops, Styracosaurus, and Centrosaurus. In each of these sister groups, you could swivel the cladogram around and preserve the relationships, while showing them in a different order. 

Cladograms can be made of both morphological (as in the features you can see from looking at a skeleton or animal) or molecular (those that come from DNA studies). Of course in palaeontology, we are restricted to morphological studies as no DNA is present.

I know that might have been a bit complicated to wrap your head around, but I think it's a really important lesson to understanding palaeontological literature, and what some of these words mean! If you have any suggestions for next week's blog, let me know!

References

1. Ryan, M. J., et al. 2012. A new ceratopsid from the Foremost Formation (middle Campanian) of Alberta. Canadian Journal of Earth Science 49: 1251-1262.

2. Farke, A. A. et al. 2011. A new centrosaurine from the Late Cretaceous of Alberta, Canada, and the evolution of parietal ornamentation in horned dinosaurs. Acta Palaeontologica Polonica 56: 691-702. -> Available free online here.