News Trip! Science on the Ancient Seas

Happy 2014 everyone!  The year is barely two weeks old, and I’ve already seen a bunch of new research about ancient marine creatures.  Apparently, 2014 is a good year for studying the seas of the past.

For today’s post, let’s take a trip back in time, and make a few stops on the way to talk about some of the new science coming out on the old oceans.

Stop #1: The Dark Waters of the Mesozoic (and Paleogene)

Let me introduce you to three creatures that were swimming the seas a long time ago:

1. A fossil sea turtle from the Paleogene Period, 55 million years ago;
2. A fossil mosasaur, an aquatic relative of lizards, from the Late Cretaceous Period, 86 million years ago;
3. A fossil ichthyosaur, a dolphin-like reptile, from the Early Jurassic Period, 190 million years ago.

Each of these creatures left behind a fossil with a rare and special feature: an ‘organic film’ containing chemical leftovers of the skin and scales that used to be there.  If you know what to look for, this film preserves information about the shape and chemical structure of pigment molecules – the molecules that gave the skin its color.

Back in 2011, I waxed poetic about the exciting new methods that were allowing paleontologists to determine the colors of ancient animals – something that many generations of scientists said could never be done.  At that time, the studies were being done on the feathers of ancient birds and dinosaurs.  Now, using very similar methods, Lindgren et al. (2014) analyzed the remains of skin and scales on those three marine fossils, and what did they find?  Eumelanin, and lots of it, from all three fossils.  Eumelanin is a pigment that gives living tissue a black or brown appearance.  All three animals were dark in color.  The next question is: Why?

From top to bottom: sea turtle, ichthyosaur, mosasaur, now drawn in
evidence-based living color!

The researchers propose two main answers to that question.

First: camouflage!  Many marine creatures today, including some sharks, rays, and whales, exhibit counter-shading, being dark on top and light on the bottom.  Viewed from above, a counter-shaded animal blends in with the dark shadows below, but viewed from below, their light bellies blend in with the sunlit waters above. We don’t know for sure, but the mosasaurs and turtle might have been counter-shaded in the same way.  The ichthyosaur, however, has remains of dark-colored skin all over its body.  In this case, the scientists compare it to the sperm whale, which is also dark all over, possibly to help it blend in with the deep dark waters where it hunts.  It may be that this ichthyosaur was also a deep diver. 

Second: heating!  All three of these animals are reptiles, which means they need to warm up in the heat of the sun.  Being dark in color would let animals like these absorb plenty of sunlight and heat to keep warm in the cool ocean waters.

Most good features of an organism have more than one purpose, so camouflage and heat-absorption are both likely good explanations.  One way or the other, this trio of ancient sea-goers can now join the ever-growing list of prehistoric animals that can be drawn in color with real evidence to back it up!

Stop #2: A 310 Million Year-Old Shark Nursery

The prehistoric 'spoonbill' shark,
Bandringa.

This time, we’re stopping to visit a prehistoric shark called  Bandringa. Bandringa lived during the Carboniferous Period, had a ‘spoonbill’ similar to a modern sawfish, and apparently raised its babies in Illinois. 

Bandringa fossils have been found at three fossil sites in Illinois, Ohio, and Pennsylvania.  Past studies have identified two different species of this shark: B. rayi and B. herdinae.  A recent study by Sallan and Coates (2014) took a close look at all of these fossils, and found two surprising things:

First, it looks like there is actually only one species of Bandringa.  What past scientists had thought were different features of two species were caused by taphonomy – the way in which the fossils were buried and preserved.  Becoming fossilized in different environments gave some of the fossils a very different appearance, but when these researchers looked past those taphonomic differences, they found that the two supposed species are actually one and the same.
Second, it looks like all of the Bandringa fossils from the Illinois site are young sharks, while the Ohio and Pennsylvania fossils are all adults.  Also interesting is that the Illinois site was formed in an ancient river delta – where a river emptied into the ocean – while the Ohio and Pennsylvania sites both represent upstream parts of the ancient river.  Coincidence?  These paleontologists think not.

In modern times, there are several species of fish, including sharks, which live their adult lives in one place, but travel to a designated ‘nursery’ to lay eggs or give birth.  This can lead to some pretty awesome migrations.  Atlantic salmon are famous for this: they typically live their adult lives in the open ocean, but swim upriver to breed in the freshwater streams of Europe and North America.

It looks like Bandringa may have done something similar, but in reverse, giving birth in the estuarine (freshwater + saltwater) environment of the ancient Illinois delta, then swimming upstream to live its adult life in the prehistoric freshwater rivers of Ohio and Pennsylvania.  On top of finding all the juvenile sharks living in the estuary, the researchers also found fossilized egg casings which may have belonged to Bandringa, or possibly to other fish also using this site as a nursery.  Not only does this give us some really remarkable insight into the life of this prehistoric shark, but this is also the oldest evidence of shark migration!

Stop #3: The Great Fish Divide, 450 Million Years Ago

About 450 million years ago, before there were any sharks or salmon, there lived some of the earliest gnathostomes (vertebrate animals with jaws).  Around this time, that ancient group split into two lineages: the bony fish (Osteichthyes), which would give rise to most modern fish, as well as all land-dwelling vertebrates; and the cartilaginous fish (Chondrichthyes), which gave rise to the sharks, rays, and chimaeras. The biggest difference between these two groups is right there in the names; one group has skeletons made mostly of bone, the other made mostly of cartilage.  But we are left with the ever-present question: Why?

To get at answering that question, Venkatesh et al. (2014) peered far back in history, not by looking at 450 million year-old fossils, but by looking at a modern animal, and the 450 million year-old clues in its genome.  Yep, it’s another new full-genome sequence, and this time, it’s the first fully sequenced cartilaginous fish: the Elephant Shark (Callorhinchus milli), a chimaera (close relative of true sharks and rays) which lives off the coast of Australia and New Zealand.

Callorhinchus milli, the Elephant Shark or
Australian Ghost Shark.  Adorable. 

To figure out what makes bony fish bony and cartilaginous fish not so much, the researchers first compared the genes of C. milli to genes from bony vertebrates.  They found that the Elephant Shark is missing a particular set of genes that all bony animals have: the SCPP gene family.  When a vertebrate embryo develops, much of its skeleton starts out as cartilage, which is then converted into bone; the SCPP genes are a big part of that process.  These researchers were even able to show that if the SCPP genes are removed from a developing zebrafish embryo, the fish will show a significant reduction in bone growth.  Clearly an important set of genes.

The SCPP gene family is thought to have developed over time through the duplication of an older gene, Sparcl1, which itself arose through duplication of an even older gene, Sparc.  When the researchers compared the genes of C. milli to genetic material from two other cartilaginous fish (a catshark and a skate), they found that all three have Sparc and Sparcl1, but are missing the SCPP genes.  It seems all Chondrichthyes lack that gene family.  On top of that, they also looked at the genes of a lamprey, which belongs to an evolutionary lineage that branched off before the big fish divide, and found that the lamprey is also missing SCPP genes.  Apparently that gene family is unique to the bony vertebrates.

Putting all of this together lets us embellish a bit on the story of the earliest jawed fishes.  About 450 million years ago, some of the earliest gnathostomes were swimming the seas, and they diverged into two major modern lineages.  The first lineage inherited their ancestor’s cartilaginous bodies, but in the second group, the Osteichthyes, some of the genes they inherited from that same ancestor mutated over time to give rise to the SCPP gene family, and as a result, much of their cartilage would harden into true bone.  This trait would then be passed on to all the bony fish of today, and all of the land-living vertebrates, including you and me.

Now, there is surely much more to this story, but this is a fascinating new piece.  It’s incredible what you can learn about the past by studying the clues in the creatures of the present!

Thanks for joining me on my journalistic journey!  2014 is already proving to be a very cool year in science.  Here's looking forward to another eleven months of exciting research!