Cracking the coelacanth code: Living version of HMNS fossil has genome sequenced

The coelacanth — a “living fossil” believed to have hardly changed over the last 300 million years — has finally had its genome sequenced by European researchers.

courtesy of wiki media
The deep-sea fish was the inspiration for the famous 1954 film Creature from the Black Lagoon and is well-represented here at HMNS, where we have three examples on display: a Devonian fossil, a Cretaceous specimen and a model like the one sequenced.

Researchers sorted through nearly 3 billion DNA bases to conclude that the coelacanth’s four fleshy fins were likely the early predecessors of limbs.

Although the coealcanth is related to early tetrapods — the first creatures to make the transition from the ocean to land — a comparison of the coelacanth genome with the DNA profiles of lungfish and other modern land-based animals led scientists to conclude that lungfish were the closer relative.

Coelacanths have been notoriously difficult to study, having been assumed extinct until an African fisherman caught the living fossil in 1938. Since then, only a few hundred specimens have been found.

Continue the investigation yourself at our Morian Hall of Paleontology, and see why this mysterious fish has kept researchers rapt for so long.

Your questions, answered: Do we know when chimaeras shifted to deep-water habitats?

Earlier this month we received a question on one of our past posts, The Ghost Sharks of the Jurassic, asking:

“Do we know when these chimaeras shifted to deep-water habitats? If predation and, in particular, the evolution and diversification of predatory species prompted their geographic transition, at what point would a sort of critical level have been reached to drive them into the deep? How many predators are too many?”

Why do “living fossils of the deep sea” so often represent lingering survivors of groups that long ago flourished in shallow water?

 
Rabbitfish

Examples: Rabbitfish (aka chimaeras), Coelacanths, Goblin Sharks, Giant Squid.

Excellent question – one that keeps evolutionary biologists awake at 2 a.m.

First thing: we never know when a clan of species invades deep water. This is why:

Sediments deposited on top of oceanic crust in deep-water – thousands of feet deep – rarely come to the surface where the layers can be seen by fossil-hunting paleontologists. Mud does form at the bottom of deep seas and fossils do form here. But such deep specimens have a low chance of being found by us.

Deep sea bottom mud is raised above sea level when continents collide and abyssal sediment is squeezed up and thrust across slabs of continental crust. There are narrow zones of such squeezed sediments – for example, in the Taconic Mountains of New York State. Here are slices of deep crust and sediment with deep-water trilobites. However, very few vertebrate and squid fossils are known from squeezed deposits.

Medium-deep sediment, up to 200 meters deep,  do form in the bottom of “epi-continental seas” like the famous  “Cretaceous Ocean of Kansas” that covered much of the central areas of North America. Such epi-continental seas  do drain away, and the bottom sediment becomes lifted hundreds or thousands of feet, so wind and water can erode valleys into the rock layers, exposing fossils. Epi-continental sea bottoms have given us 90% of our marine vertebrate and cephalopod fossils.

Coelacanth fossils are common in shallow-water and medium-deep sediments beginning in the Early Devonian, over 400 million years ago. From then on, coelacanths remain widespread and often common. Were they in deep water too? We don’t know – we don’t have enough deep sediment exposed for study.

Abruptly, coelacanths disappear from epi-continental sea deposits in the Late Cretaceous. Naturally, we thought they were extinct. But then the fish show up alive and well, hanging around at 130 meters to 700 meters.

Ditto for the Goblin Shark: common as a fossil along New Jersey in shallow sediment but now restricted to much deeper waters. Ditto for the giant squid, who left their shells in the Cretaceous epi-continental sea sediment but now prefer deeper water.

Goblin Shark

Is there a common explanation for all the survivors in deep waters?

The most popular theory is: 1) Most new types of fish and cephalopods first evolve in shallow water. 2) It takes time for evolution to modify a fish or cephalopod so the beast can survive at 200m + depths. So the early coelacanths couldn’t colonize the great depths for tens of millions of years. As more and more clans of fish evolved in shallow water, some began their adaptive descent too – but the coelacanth had a head start. Being fully adapted to great depth already may have protected the fish from predators and competitors who are behind in the degree of their transition.

There are holes in the theory. Coelacanths do have predators – they show up in shark stomachs. They must have competitors too – teleost fish with more complex jaws.

Deep Sea Refuges continue to irritate our neat little hypotheses.

WAnt more? See the past post on ghost sharks and full comment.

100 Years – 100 Objects: Life Through Time Mural

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from David Temple, the museum’s curator of paleontology. He’s chosen a selection of objects that represent the most fascinating fossils in the Museum’s collections, that we’ll be sharing here – and at 100.hmns.org/ – throughout the year.

life through time muralTimelines are an important element to telling a story through exhibits. At the Houston Museum of Natural Science, early life on Earth is recreated in a linear timeline, with the fossils themselves as the “stops.” The time represented is collectively grouped as the Paleozoic period, and accounts for around 300 million years of the history of life.

The fossils displayed on the wall form the stops on the time line, grouped by famous localities that historically defined the sub eras: Pre-Cambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. This is appropriate as, prior the development of technology that allows paleontologists  to compute absolute dates, the divisions between the geologic periods were defined by the fossils present in geologic layers. At first glance, the fossils in the case may appear similar, but they are very different, and come from many of the type localities that created the boundaries.

To bring this timeline to life, a vibrant visual motif was created. Mounted behind the fossils is a brightly lit graphic that unfreezes the animals from their rock prisons and remembers them as best we can, alive and in their natural environments. This motif came from an original oil painting commissioned by the Houston Museum of Natural Science. The Paleozoic diptych, painted by acclaimed paleontology artist William Stout, is titled “Life Before the Dinosaurs.” The original oils have never been exhibited publicly, but grace the walls of a conference room.

Wander among prehistoric beasts in the Paleontology Hall, a permanent exhibition at the Houston Museum of Natural Science.

You can see more images of this fascinating artifact – as well as the others we’ve posted so far this year – in the 100 Objects section at 100.hmns.org.

100 Years – 100 Objects: Sponge encrusting a Cephalopod

The Houston Museum of Natural Science was founded in 1909 - meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.

This description is from David Temple, the museum’s curator of paleontology. He’s chosen a selection of objects that represent the most fascinating fossils in the Museum’s collections, that we’ll be sharing here – and at 100.hmns.org/ – throughout the year.

Sponge, Protospongia rhenana encrusting a cephalopod, Orthoceras
(Devonian, Hunsruck, Slate, Bunden bach, Germany)
HMNS 498

CHI_7601L resizeThis sponge, Protospongia rheana is an example of a delicate animal preserved after being drowned in oxygen-poor silt 390 million years ago. A sessile animal, this sponge attached to a hard spot on the sea floor. On this example, it attached itself to the hard remains of the cephalopod, orthoceras. This sponge may have been buried in place in, or broken free of, the seabed and swept by currents along the bottom before settling in a low area, its final resting place.

Wander among prehistoric beasts in the Paleontology Hall, a permanent exhibition at the Houston Museum of Natural Science.

You can see more images of this fascinating artifact – as well as the others we’ve posted so far this year – in the 100 Objects section at 100.hmns.org.