The Archosaurs: A Brief Summary of Reptilian Evolution

320 million years ago, during the Carboniferous Period, the flora and fauna looked quite different than it does today.  Oxygen levels in the atmosphere were much higher, allowing insects to grow to enormous sizes.  Centipedes were the size of snowboards; dragonflies were the size of hawks; and spiders were the size of dinner plates.  There were also no mammals.  Not only that, but there were no reptiles, no dinosaurs, and no birds (which, of course, are dinosaurs).  There were, however, amphibians.  Amphibians, like frogs and salamanders, cannot lay their eggs on dry land, and instead must lay them in pools of water.  This is because the eggs that they lay, unlike those laid by dinosaurs, birds, reptiles, and monotremes (the two egg-laying mammals), have soft-shelled eggs, which allows water to move in and out of them.  They would simply dry up on land!

Around 320 MYA, however, something changed.  Some of these amphibians developed what is known as an "amniotic egg," the type of egg typified by the dinosaurs, birds, reptiles and monotremes.

Around 5 million years after that, around 315 MYA, another major split occurred, this time between the amniotes.  This split resulted in two lineages, the first of which, the synapsids, would one day become the mammals.  The second of these two lineages was the sauropsids.   Within the sauropsids was the group known as the archosaurs.  The archosaurs, in turn, suffered two major splits.  The first split was the crocodiles, alligators and their kin.  They joined with the rest of the reptiles contained within the sauropsids.  The second large split within the archosaurs was another large group, off of which the pterosaurs broke off, before the rest of the group became dinosaurs (and, in turn, birds).  If you look at the family tree below, you can see that the group known collectively as the "reptiles" is what is referred to as a "paraphyletic group."  Put simply, that means that a paraphyletic group consists of "all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants, typically just one or two such groups." So for reptiles, that means that the group consists of the last common ancestor of all reptiles (which includes both extant reptiles and the extinct mammal-like reptiles, the precursors of the mammals), including all of the descendants of that ancestor....EXCEPT for the mammals and the birds, which are defined separately.  I feel like I have confused you enough, so I am not even going to get into the whole dinosaur debate.

The Fauna of South Carolina: Reptiles and Amphibians

First off, don't forget to look below to see the answer to last weeks mystery animal, and to see a couple of pictures of this weeks!

Recently, my family visited some good friends of ours in South Carolina for a few weeks.  Not only did we have a lot of fun visiting them, we saw a lot of cool plants and animals there that we simply don't have up north!  For some of you, many of these animals and plants will be routine and boring, but hopefully there will be something in here that you will find interesting!  I think I am going to split this post up into at least a few different segments, with tomorrows post devoted entirely to the Black Skimmer, a very interesting bird that I had the good fortune to see hunt!  So today I am going to talk about some of the interesting reptiles and amphibians we saw down there, and over the next few weeks we can take a look at some of the birds, fossils and other things we saw down there!

Due to the increased heat and humidity, coupled with less of a swing between the seasons (i.e. it rarely snows and drops below freezing), the south is an excellent place to see all sorts of reptiles and amphibians.  We saw a number of turtles and a frog/toad or two when we were in the south, but what interested me more were the lizards.

A Carolina anole (Anolis carolinensis).  Despite its name, the Carolina anole is found in both South and North Carolina, as well as Florida, Georgia, Louisiana, Texas, Alabama and Mississippi.

A type of skink, but I have yet to ascertain its species, although I am almost certain its genus is that of Plestiodon

And then, of course, there's the gators.  The American Alligator lives in the same states as the Carolina anole, as well as Virginia, Arkansas and Oklahoma.  While we did see a few in the wild, all of these shots (except for that of the gator footprint) were taken at a place called Brookgreen Gardens, or at a restaurant called the Crab Shack.

This big gator we saw at Brookgreen Gardens

This is a picture of a gator footprint that I took.  You can see three of the toes very well, making it look a lot like a theropod dinosaur track, but you can see the other two toes as little holes in the ground.

This picture, and all the rest, were taken at an awesome (both food- and entertainment-wise) restaurant on Tybee Island in Georgia called The Crab Shack.

 

 

 

 

 

 This post is part of "The Fauna of South Carolina" series.  For the rest of the posts in this series, click HERE.

Sea Pines on Dwellable

The Biggest Carnivorous Dinosaur Part 3: Spinosaurus, The World's Largest Penguin

Spinosaurus quickly smashed its way to the top (literally!) following the success of Jurassic Park III.  At up to 56 feet (17 meters) in length, Spinosaurus (found in Egypt and Morocco) is unquestionably the longest carnivorous dinosaur that has ever been discovered, and by a significant margin, too.  But how could Spinosaurus be so successful when another carnivorous monstrosity, Carcharodontosaurus, lived in such close proximity?  The answer lies in what it ate; in fact, Spinosaurus appears not to have been as carnivorous as we once thought, and seems to in fact have been more piscivorous.  Essentially, it is thought that Spinosaurus might have survived on a diet consisting a great deal upon fish.

There is a great wealth of information that indicates the staple diet of Spinosaurus was fish.  Here we will take a more in-depth look at this evidence.

A picture of Spinosaurus drawn by the talented young artist Sam Lippincott.

1.  POSITION OF THE NOSTRILS:  Spinosaurus, unlike most other theropod (meat-eating) dinosaurs had nostrils situated very high up on its skull.  By contrast, most other theropods, like Tyrannosaurus, had nostrils very close to the front of the skull.  The nostril placement in Spinosaurus would have allowed it to stick its nose into the water quite deep, enabling it to hunt fish more effectively.

2.  SENSORY ORGANS ON THE TIP OF THE SNOUT:  Along with the high placement of the nostrils, Spinosaurus also had a number of small sensory organs at the front of the snout, revealed by CT scans.  These greatly resemble those of the crocodile, who uses them to sense where prey is by "seeing" tiny movements in the water.  Thus, a crocodile can attack its prey without ever seeing them with their eyes.  Spinosaurus appears to have been able to do much the same thing, making its hunting method of choice quite obviously effective, given its immense size.

3.  ISOTOPIC EVIDENCE:  A study of the isotopes in the skull of a Spinosaurus specimen revealed something quite interesting.  Spinosaurus shares a great deal of isotopic overlap with crocodiles, fairly aquatic animals.  By contrast, other carnivorous dinosaurs like Tyrannosaurus have very different isotopic readings, indicating a terrestrial lifestyle.  Given the great deal of overlap possessed by Spinosaurus and crocodiles, it appears that Spinosaurus spent much of its life like a crocodile; semi-submerged in rivers and streams.

4.  THE TEETH TELL THE TALE:  Spinosaurus, unlike most other carnivorous dinosaurs like Tyrannosaurus, Allosaurus, or the famous Velociraptor, possess conical, round teeth.  These other carnivores possess thin, blade-like teeth, made for slicing, crushing or tearing into other animals.  Spinosaurus has teeth much like a crocodile; round and conical.  These teeth are made for gripping and making sure that an animal does not escape. 

5.  THE TEETH TELL THE TALE....AGAIN:  Yet again, we come to the issue of teeth, and this is one of the most damning pieces of evidence of all.  During the time that Spinosaurus lived in North Africa, another inhabitant of these streams and rivers was a saw-toothed fish called Onchopristis.  Two fossils involving Onchopristis have been discovered in conjunction with the remains of Spinosaurus.  One was the skull of a Spinosaurus; imbedded in one of the tooth sockets was a vertebrae from an Onchopristis.  The other discovery was the barb of an Onchopristis embedded in the jaw of the Spinosaurus.

6.  PLUS:  We only ever seem to find Spinosaurus in shallow aquatic environments, where fishy prey would be abundant.

All in all, it seems like an inevitable conclusion to draw that Spinosaurus was, in fact, a piscivore, at least much of the time.  A fascinating example of the crazy turns evolution can take.