Cetartiodactylais the clade in which whales (including dolphins) and even-toed ungulates have currently been placed. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla reflects the idea that whales evolved within the artiodactyls. Under this definition, their closest living land relative is thought to be the hippopotamus. The clade formed by uniting whales and hippos is called Whippomorpha. Alternatively, the term ‘Cetartiodactyla’ is used to denote a clade where Cetacea evolved alongside Artiodactyla and not within it. Under this definition, all artiodactyls, including hippos, are more closely related to one another than any are to the whales.
Elomeryx, top, was a land animal related to modern-day goats, pigs and hippos. Pakicetus was clearly a water creature, but it spent some of its life on land and had the feet of a land mammal. Rodhocetus’s feet worked for both walking and swimming. Dorudon is striking for its resemblance to modern whales (note the front flippers and horizontal flukes)—but it still sported tiny back feet.
[Image: S.H.Morgan’s cladogram of the animal groups that fall under Dinosauria. Somewhat outdated, but still useful for our purposes.]
Many of us grew up referring to any vaguely reptilian prehistoric animal as a “dinosaur”. In truth, that group is much more exclusive than you might think. Dinosaurs fall under one of two orders—Saurischia or Ornithischia—and share a more recent common ancestor with one another than with any of the following animals:
The first vertebrates capable of powered flight, pterosaurs ruled the Mesozoic skies long before the earliest birds appeared on the scene. Current thinking is that they shared a close relationship with dinosaurs in the group Ornithodira, but they themselves were not dinosaurs.
Many prehistoric crocodile relatives had erect limbs like dinosaurs, so perhaps it’s no wonder people get them confused. However, these animals evolved their erect stance independently of dinosaurs. A good rule of thumb to remember is that if it walks on four legs and looks like a crocodile, it probably isn’t a dinosaur.
Unlike birds such as penguins, non-avian dinosaurs generally weren’t as big on the whole aquatic lifestyle thing as we once thought. The dolphin-like ichthyosaurs, long-bodied mosasaurs and snaky- or thick-necked plesiosaurs were more closely related to lizards than to dinosaurs.
As synapsids, the often sail-backed pelycosaurs were more closely related to mammals than to dinosaurs. That’s right—creatures often marketed as dinosaurs actually occupy a branch on the animal family tree much closer to you and me!
This group includes modern mammals, so it should be pretty obvious why they’re not considered dinosaurs despite many of the early forms’ more reptilian appearances.
Plenty of other examples exist, but these critters are some of the most common culprits when it comes to being confused for dinosaurs. Remember, it can’t hurt to do your research before calling something a dinosaur!
150 Million Years of Fish Evolution in One Handy Figure
Have you ever wished you could have the entire 150 million years of spiny-rayed fish evolution in convenient poster form? Well, wait no longer. Your happy day is here! Trust me … this is one poster featuring mullets you will not be embarrassed to display.
Plus, you can use it as a way to explore diversity and procrastinate. Not feeling like work? Simply glance over and … morwongs?! How did I never know such things existed? Maybe I’ll just have a quick … [2 hours pass]
In this figure, you can see how all the spiny-finned fish (acanthomorphs) — more than 18,000 species of them, which represent nearly one-third of living vertebrates — are related to one another. To accomplish this, the scientists inferred the relationships from the sequences of 10 genes from 520 spiny-rayed fin fish representing most of their families. They combined this data with that of 37 fossil “age constraints” used as reality checks on the actual timing of evolutionary shifts.
So that you can savor appropriately, it’s worth noting that this section of the vertebrate family tree has evidently given scientists significant difficulty, for the team who created it write in July in the Proceedings of the National Academy of Sciencesthat this group of fish has long “remained the last frontier” in drawing the family tree of living vertebrates and that it has presented “one of the most unyielding problems in vertebrate phylogenetics”. Blood… sweat… probably tears went into the creation of this image. Which makes its arrival all the more a moment to appreciate.
These prickly-finned fish include most of the fishes you think of as fish, with several major exceptions including sharks, sturgeon, trout, and salmon. They include fish that live in high mountain lakes and at the bottom of ocean trenches, fish flat as a pancake and puffy (though not fluffy) as a pillow. As advertised, they all have characteristic sharp, bony spines in their fins that any angler can tell you that you must mind when removing a squirming, slippery, and likely panicked fish from the hook.
The scientists found that the group likely evolved in the Early Cretaceous, about 150 million years ago. It’s a bit weird to think about what that means — that the tropical reefs of the Age of Dinosaurs — and all fish-containing ages prior — contained none of the tropical reef fish we recognize today (but what wonders did they contain?).
In the past, scientists wondered what circumstances or events led this group to evolve so much. Some suggested coral reefs were the cradle of the group’s diversity. But in the PNAS study, the five lineages within the group that diversified the most are found in four very different habitats: freshwater (cichlids), open ocean (tuna and friends), cold temperate seafloor (snailfishes), and coral reefs (blennies and gobies). This seemingly confounds any simple explanation for the massive success of the group.
The scientists also found that this group of fish didn’t seem to diversify much in response to the End-Cretaceous Mass Extinction. You can see this in the figure above. The dashed circle is when the Really Big Rock Hit Planet Earth. There doesn’t seem to be a big burst of evolution in its aftermath — that is, a whole lot of new lineages created all at once, perhaps to fill niches vacated by the Really Big Rock. Although it’s well established that many groups of snakes, lizards, birds, and mammals diversified significantly in response to the asteroid strike (although some major groups of these animals did not), it appears that this giant piscine group of vertebrates remained relatively unaffected by what was for many terrestrial vertebrates an Earth-shattering opportunity.
Near T.J., Dornburg A., Eytan R.I., Keck B.P., Smith W.L., Kuhn K.L., Moore J.A., Price S.A., Burbrink F.T. & Friedman M. & (2013). Phylogeny and tempo of diversification in the superradiation of spiny-rayed fishes, Proceedings of the National Academy of Sciences, 110 (31) 12738-12743. DOI:10.1073/pnas.1304661110
Helicoprion is a genus of prehistoric shark-like fish that lived from the late Carboniferous to the Permian mass extinction, 310 to 250 million years ago. They are distinguished by spiral clusters of teeth, called ‘tooth whorls’. These structures may have functioned to allow the teeth to be continually regenerated throughout life. Because these animals had skeletons made of cartilage, not much else in known about their body structure or lifestyle.
February on this blog is going to be Daily Paleo Art Month! Because doing dinosaurs all last July was so much fun I want to do this thing again. Every weekday for the rest of the month I’ll be posting a new image of something strange, obscure, or just plain interesting from the fossil record — only this time we’re staying firmly outside of the Avemetatarsalia (pterosaurs and dinosaurs/birds) to give some less famous critters the spotlight.
A cartilaginous fish from off the southwest coast of the ancient supercontinent Gondwana (and later Pangaea), Helicoprion first appeared in the late Carboniferous (310 million years ago) and survived up until just past the massive Permian-Triassic extinction (250mya). Despite looking rather shark-like and possibly reaching sizes of around 6m (20ft) long, it was actually closer related to the chimaeras.
For a long time, the only parts of this animal known were bizarre buzzsaw-like spiral whorls of teeth, since cartilage skeletons very rarely fossilize. The ideas for just where in the body this structure was positioned were ridiculously varied.
The most recent reconstruction is based on CT scans of a well-preserved fossil with jaw and skull elements, which showed the whorl taking up the whole lower jaw. It also turns out Helicoprion had no upper teeth at all. It’s thought to have used this arrangement to shred and crush up squid and other soft-bodied marine prey, but there’s still very little known about how such a unique type of teeth evolved in the first place.
A Dunkleosteus skull, photographed at the Royal Tyrrell Museum in Drumheller Alberta. Dunkleosteus was a placoderm fish that lived in the late Devonian, 380 to 360 million years ago. It was a hypercarnivorous apex predator, feeding on armored prey such as ammonites, arthropods, and other placoderms. Fully grown individuals had more than 700 kilograms of bite force, enough to easily shear through bone and protective tissues. Members of the largest species could grow up to 10 meters in length and weigh almost four tonnes.