Coelacanth: Fossil Record

Summary: Join Kiersten as she takes a trip through time with the fossil record of the coelacanth.

 

For my hearing impaired listeners, a complete transcript of this podcast follows the show notes on Podbean

 

Show Notes:

“Coelacanth Fish Fossils, Mawsonia Woodward, 1907,” by Prof. Dr. Sc. Norman Ali Bassam, Ali Taher, Mohammad Ahmad, Mostafa Khalaf-Prinz Sakerfalke von Jaffa. https://issuu.com

“The first late cretaceous mawsoniid coelacanth (Sarcopterygii: Actinistia) from North America: Evidence of a lineage of extinct ‘living fossils’.” By Lionel Cavin, Pablo Torino, Nathan van Vranken, Bradley Carter, Micheal J. Polcyn, and Dale Winkler. PLOS ONE, https://journals.plos.org

“Fossils of Cretaceous-Period Coelacanth Discovered in Texas,” by Sergio Prostak, SciNews, November 16, 2021. https://www.sci.news

“Oldest coelacanth, from Early Devonian of Australia,” by Zeroing Johanson, John A. Long, John A Talent, Phillipe Javier, and James W. Warren. Bill Lett, 2006 Sep 22; 2(3): 443-446; doi: 10.1098/rsbl.2006.0470

“Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian,” by Min Zhu, Xiaobo You, Jing Lu, Too Qiao, Wenjin Zhao, and Liantao Jia. Nature Communications 3, Article Number: 772 (2012) https://doi.org/10.1038/ncomms1764

“Ghost Lineages,” by Matt Wedel, 5/2007 and 5/2010. https://ucmp.berkeley.edu

 

Music written and composed by Katherine Camp

 

Transcript

(Piano music plays)

Kiersten - This is Ten Things I Like About…a ten minute, ten episode podcast about unknown or misunderstood wildlife.

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Welcome to Ten Things I Like About… I’m Kiersten, your host, and this is a podcast about misunderstood or unknown creatures in nature. Some we’ll find right out side our doors and some are continents away but all are fascinating. 

This podcast will focus ten, ten minute episodes on different animals and their amazing characteristics. Please join me on this extraordinary journey, you won’t regret it.

This episode continues the coelacanth and the ninth thing I like about this animal is its fossil record.

Throughout this series I’ve talked about the fossil’s of the coelacanth and how they are sometimes called a ‘living fossil’, so I thought we should take a few minutes to look at their actual fossil record.

As we have discussed before coelacanths are old. The first coelacanths lived about 400 million years ago in the Devonian period. This was approximately 170 million years before dinosaurs roamed the earth. No matter how many times I say it, it still blows my mind!

The fossil record of the coelacanth, just like everything else about this fish, is actually quite interesting. Throughout their long history coelacanths have been thought to be evolutionary conservative which essentially means they haven’t changed much, but when we look a litter closer at the various fossils we see a different story. Our modern living coelacanths look like something that swam right out of ancient history, but throughout their existence they have had several body shapes. 

Let’s look at the Devonian coelacanths. The best known Devonian coelacanth fossils come from the late Middle to early Late Devonian period. There are two early coelacanths that are well known, Gavinia and Miguashaia. These two genuses are considered primitive coelacanths because they are more like primitive lungfish and less like modern coelacanths in body form. What researchers look at to determine these classifications are the skull shape, the fin placement, and the tail. 

If we compare the skull shapes, in layman’s terms, of Miguashaia and Latimeria (as a reminder that is our modern coelacanth) the Devonian era  coelacanth’s skull is broader and shorter, the body is shorter and more stout, and the tail is dramatically different. The Miguashaia tail technically has three parts like the modern coelacanth but the top fin is tiny while the bottom fin is much larger. The puppy dog tail portion of the tail that runs between the two fins sort of curves up a bit. The majority of the tail fin is below the midline and is square as opposed to the rounded tail of Latimeria. 

These are the most well known fossils from the Devonian period and they are fully formed enough that they can be placed in the coelacanth timeline based on body shape. But these are not the only fossils found from the Devonian era. There were fossils found in Australia from the early Devonian period suggesting coelacanths are even older than we previously thought. Researchers are hesitating to use these fossils when phylogenetically classifying coelacanths because it’s only a lower jaw bone. The existence of a dentary sensory pore in the jaw proves it is a coelacanth, as modern day coelacanths, as well as other fossils throughout the ages, have dentary sensory pores also.

Now, there have been approximately 80 species of coelacanth fossils described from the Middle Devonian to the Late Cretaceous. The Late Cretaceous dates from 360 million years to 70 million years ago. In the Cretaceous period, two families of coelacanths are represented through the fossils that we have found. One is Latimeriidae and Mawsoniidae. 

A scientific paper published in 2021, discussed the discovery of Cretaceous period mawsonid coelacanth fossils found in the Woodbine Formation in northeast Texas. The reason these fossils are important is that they expand the regional location of coelacanths. These are the first coelacanth fossils found in North America. We didn’t know that they lived in the area of North America until these fossils were found. Researchers postulate that these coelacanths got here during the break-up of Pangea, but we need a lot more research before we have any solid theories.

One of the things I wanted to know about ancient coelacanths was how big they were. It seems like when we go back in time, animals are always bigger than they are now. Like the dragonflies that used to be as big a VW Bug, sloths that were the size of an SUV, and sea scorpions the size of small sedan. Well, some of the coelacanth fossils that we have found are complete bodies and some have enough bones to extrapolate how big the fish was when they were alive. So we have a range from about two feet to thirteen feet! Our modern coelacanths seem to have settled somewhere in the middle. 

Coelacanths were believed to have gone extinct during the Late Cretaceous period. Today we know that’s not true, but until 1938 we hadn’t seen any or more importantly, we hand’t found any younger fossils. The last record we had of the coelacanth came from the Cretaceous period. You may be wondering how this is possible, I know I was when I started researching this episode. I found a great article from UC Berkley that helped me understand what happened to the missing evidence of coelacanths for the last 60 million years. 

Lineages are important when studying the fossil record of any living things. Lineages are the unbroken chains of ancestors and descendants. They tell us who is related to whom. A ghost lineage occurs when a line of descent leaves no trace in the fossil record. This is what has happened to our beloved coelacanth. Now back to our question, how is this possible? How come we can’t find fossil evidence of the coelacanth after the Cretaceous period.

Living coelacanths reside in deep ocean waters near volcanic islands. To create fossils, whatever dies is preserved by layers of sediment and then exposed million of years later. If you are a deep water resident your fossils have to rise above sea level and eventually become exposed in an area where humans can find it, whether through natural erosion or paleontological digging. Well, most fossils are more than 70 million years old, so we haven’t found younger coelacanth fossils yet because they’re still hidden in the depths of the ocean where our modern coelacanths live. 

Coelacanths are considered a Lazarus taxon. A Lazarus taxon is a group of living beings that reappear after a long period during which they were thought to be extinct. The name is based on the biblical story of Lazarus who was raised from the dead. There are typically two characteristics shared by Lazarus taxons. 1- They have a limited geographic range. 2-They live in an area where fossils rarely form. This certainly sounds like the coelacanth to me. 

That is all for this penultimate episode of the coelacanth. The fossil record of this majestic fish is my ninth favorite thing about this long-lived animal.

 

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Join me next week for the final episode about the coelacanth.  

 

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This has been an episode of Ten Things I like About with Kiersten and Company. Original music written and performed by Katherine Camp, piano extraordinaire.