Quetzalcoatlus - the largest pterosaur 

Reconstructions by Mark Witton. 

When: Late Cretaceous (68-65 million years ago)

Where: North America

What: Quetzalcoatlus is a gigantic pterosaur. Just how gigantic it was has been the subject of some debate, as no 100% complete specimen has been found. While the first estimates put its wingspan at up to 50 feet (16 meters) this has been reduced to 36 feet (11 meters) in the latest studies. The reason for this disparity is due to allometry - the physical properties of bones require that as an animal gets larger its skeletal structure is not just that of a smaller animal made larger. Thus the wing bones of Quetzalcoatlus were relatively thicker than that of a smaller species, and while this was taken into account in the first estimates, it took a better understanding of pterosaur evolution in general for a refined estimate to be generated. 

This large size brings with it another debate: could Quetzalcoatlus fly? The answer is yes, this pterosaur sailed over prehistoric Texas. A big mystery was how Quetzalcoatlus could take off, and recent work by functional morphologists has provided a solution to this puzzle. Pterosaurs differed from all other flying vertebrates in that they retained the majority of the digits on their hand outside of the wing itself; this not only allowed these fingers to be used to manipulate their environment, but was critical for terrestrial locomotion. Quetzalcoatlus was quadrupedal on the ground, like all other pterosaurs, but it had a specially developed system of ligaments and tendons in its wrist joint that allowed it to ‘spring’ up and take flight.  This can be seen in this video.  

Another, more minor, debate is what did Quetzalcoatlus eat? Most pterosaurs are closely associated with large bodies of water and have a fish based diet - but all Quetzalcoatlus remains have been found hundres of miles from ancient shorelines. This, combined with morphology of the skull, has lead to the conclusion that these giants instead fed on smaller vertebrate that they would capture with their large beaks, such as the baby sauropod not having a good day in one of the reconstructions above. 

Styracosaurus

Both mounted specimens on display at the American Museum of Natural History, NYC

When: Late Cretaceous (~75 Million years ago)

Where: North America, most fossils known from Alberta and Montana. 

What: Styracosaurus is a ceratopsian dinosaur. It is easly distinquished from its more famous relative Triceratops as it lacks the two large brow horns, instead having four to six spike-horns extending from its fenestrated frill. It is also a bit smaller than this giant ceratopsian ‘only’ reaching lengths of 18 feet (~5.5 meters). Styracosaurus material was first collected by the great fossil collector C.M. Sternberg in 1913, from what is now Dinosaur Provincial Park in Alberta, Canada. This area proved to be a treasure trove of Styracosaurs material and has been visited by fossil collectors repeatly over the decades. The large amount of material of this one dinosaur has supports the conclusion that Styracosaurs traveled in herds, as has been proposed for other ceratopsians. 

Ceratopsians appear to have originated in Asia in the Jurassic period, but it was after the migration of members of the clade to North America that they greatly increased in size and developed the large neck frills and skull ornamentation the group is famous for. There has been much debate as to the purpose of the horns and frills. The modern consensus is that they functioned for defense from predators, combat between individuals, and for species identification for mating.  

Hesperornis 

When: Late Cretaceous (~85-70 million years ago) 

Where: Mostly in the Cretaceous Seaway of North America, but there are also fossil finds in Russia

What: Hesperornis is a member of the first lineage of aquatic birds, and the only aquatic birds (or any dinosaur for that matter) in the mesozoic. Hesperornis was arguably more adapted for swimming than the modern penguin. These birds were so modified for the marine realm that they could not even stand upright, and instead on land had to shuffle forward on their bellies, like the modern seals. This is because their primary means of propulsion in the water was their feet, not their wings/flippers. To accomplish this, their hip joint shifted so that their legs extended out laterally from their body, rather than downward. It is thought that their feet were not webbed, but instead they had expanded lobes on each of their toes, as in the modern grebbe. Hesperornis forelimbs were highly reduced and only functioned as rudders.  These birds were fairly large, reaching up to 5 feet (~1.5 meters) long. 

The clade containing Hesperornis is the oldest known example of flightless birds, and spilt from the lineage leading to modern birds fairly quickly, as shown by their retention of such features as teeth in the jaws. It is unclear exactly where Hesperornithiformes diverge from the greater avian evolutionary tree, but there is consensus that they are not within Neornithes, the clade that contains all extant birds and their fossil relatives.  

Archelon 

When: Late Cretaceous (~75 to 65 million years ago)

Where: The shallow sea that covered Central North America during the late Cretaceous.  

What: Archelon is an extinct sea turtle. It is the biggest sea turtle known; the largest specimen measuring 13 feet (~4 meters) long and 16 feet (~5 meters) wide, including the flippers. Archelon’s weight is estimated at almost 5,000 lbs (~2,200 kg). The closest living relative of this massive reptile is the leatherback sea turtle, Dermochelys.  As a comparison, the largest living leather back on record was 10 feet (~3 meters) long and weighed over 2,000 lbs (~900+ kg). As in the leatherback today, Archelon did not have a completely solid shell, but rather had large open areas in the bones, which were covered by thick skin. This results in a far lighter shell than if the structure was solid. This reduction in bone mass is part of a suite of adaptations that allowed these turtles to grow so large. Archelon is thought to have had a long life span, based on growth lines on long bones, several specimens have estimated ages of over a century. 

Archelon swam though the Cretaceous Seaway, a shallow sea that covered central North America in the late Cretaceous. It shared these waters with many large reptilian predators, such as the mosasaurs, an example of which is seen behind the mounted skeleton in the top image above.  As you can see these carnivores were plenty large enough to easily make a meal out of this huge turtle. Archelon and all members of the family Protostegidae vanish from the record at the end of the Cretaceous period. 

Tylosaurus 

When: Late Cretaceous (90 - 80 million years ago)

Where: Swam mostly though what is now the great plains region of North America, with the best fossils to date found in Kansas. 

What: Tylosaurus is a giant lizard. No really! It is a mosasaur, which were a group of marine predatory lizards that swam the warm seas of the late Cretaceous. Mosasaurs as a whole are most closely related to varanids (monitor lizards, including the Komodo Dragon)  within Squamata and are the biggest lizards to have ever existed. Tylosaurus was one of the largest mosasurs, estimated to have reached over 50 feet (~15 meters) long. It’s skull alone was roughly 5 feet in length! It was an apex predator, feeding on anything and everything it could snap between it’s giant jaws. A specimen has been found that had plesiosaur remains inside its ribcage. 

This humongous beast swam though the Cretaceous Seaway aka the North American Inland Sea. This was a huge, warm, shallow sea that ran north-south though North America during the latter half of the Cretaceous, splitting the landmass in half. The sea was over 600 miles (~1000km) wide, almost reaching from the Rockies to the Appalachian mountains, and is estimated to have been 2,500 feet (800 meters) deep, which is relatively shallow for such a large body of water. Tectonic activities near the end of the Cretaceous caused this sea to vanish as quickly as it formed, resulting in the extinction of giants like Tylosaurus. 

Parapuzosia - the giant ammonite 

When: Late Cretaceous (~100-65 million years ago)

Where: Northern Hemisphere 

What: Parapuzosia is the largest of the ammonites! Like the rest of it’s clade it vanishes from the rock record at the end of the Cretaceous period, in the same extinction event that took out the dinosaurs. Ammonites are molluscs, putting them in the same group as snails, clams, squids, etc. Within Mollusca they are in celphalpoda along with octopuses, squids, cuttlefish, and nautiluses. Though ammonites strongly resemble nautiluses, they are more closley related to the less shelly taxa in this clade. As mentioned before Parapuzosia is HUGE. How huge? The shell measures 8 feet (~2.5 meters) across. And that is just the shell! The body and all its tentacles would have extended from the carapace by almost this much again.  This fossil is even not that rare! Its known from thoughout the northern hemisphere, though most of the specimens are fragmentary. The amazing almost complete specimen above was collected in Germany. 

Parapozosia, like all ammonites, was carnivorous. It ate fish and anything else it could get its tentacles on. They were pelagic, meaning they freely swam around the ocean. As they were so large each individual probably had a very large geographic range. The clade Ammonoidea had an extremely large temporal range. Fossils are found from the Devonian (~400 million years ago) to the end of the Cretaceous (~65 mya). These fossils are so common and well preserved that we use them today to help date different rock layers. They also have a very long modern scholarly history; there are records of romans writing about ammonites! Here I will shamelessly end with a fact from wikipedia: “Pliny the Elder called  fossils of these animals ammonis cornua (“horns of Ammon”) because the Egyptian god Ammon was typically depicted wearing ram’s horns. ”

Pakasuchus - the cat-like crocodile

When: Middle Cretaceous ~ 105 million years ago

Where: Tanzania, Africa

What: A Notosuchia Crocodylomorpha. Crocodiles today are all aquatic, sprawling, and fairly large (the smallest is about 5ft/1.5 meters long). However, fossil crocodiles occupied many niches and had a wide range of body types not seen in living species. A great example is Pakasuchus. This small and agile croc was completely terrestrial and only about the size of a house cat!  Its dentition was also very different from modern crocs. Living crocodiles are all homodont, all of their teeth are fairly similar cones, but Pakasuchus was heterodont like mammals. Its had not only cone like teeth, but also some teeth well suited for shearing and others for crushing and grinding. 

Pakasuchus and the rest of the notosuchians, most of which are cool enough they will be individual highlighted in future posts, are excellent examples of how the fossil record can show us lost diversity within clades that today are fairly homogenous. Crocodile line archosaurs (the clade containing crocs and dinos) were hit hard by the end Cretaceous extinction, with only modern forms surviving.  Its probable Pakasuchus or animals much like it survived to the end of the Cretaceous, even though we only have a handful of fossils from the mid-Cretaceous. Careful inspection of fossil material for distinctive Pakasuchus teeth needs to happen, and will hopefully increase it’s temporal and geographic range.