Evolution -- Sharks -- December 6, 2024

Books:

• Evolution: The Whole Story, General Editor, Steve Parker; Foreword by Alice Roberts, Thames & Hudson.
• The Secret History of Sharks: The Rise of the Ocean's Most Fearsome Predators, John Long, c. 2024

 

Lessons learned:

• a better understanding of mass extinctions
• sharks -- one of the longest living vertebrates?
  
• years to evolve: 465 million years and counting
• is there any vertebrate that's been around longer?
  
• remarkable for ability to survive the huge mass extinctions, changing environments; changing predators (p. 11)
• cartilaginous -- unique; more flexible than bony fish
• diverse species; unique niches
• evolution: homeobox genes probably more important than mutations -- p. 11 -- homeobox discovered 1984 (I graduated from high school, 1969; college, 1973; and, medical school, 1977.

The Age of Fish

Paleozoic ERA
    Permian                        jawless (agnathans)
    Ordovidian -- first fossils of oldest shark? -- one example
    Silurian -- one example
    Devonian -- appearance of teeth and teeth replacement like modern sharks
    Carboniferous             

        Hagfish
        Lampreys

Only surviving jawless fish: hagfish and lamprey

Mesozoic -- Age of Dinosaurs (ERA)
    Triassic period
    Jurassic period
    Cretaceous period

 

Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Sub Phylum: Vertebrata
Infra Phylum: Gnathostomata -- jawed vertebrates -- 99% of all living vertebrates (incl. humans)
Clade: Eugnathostomata
Class: Chondrichthyes

 

Infraphylum of jawless fish: Agnatha
    so, Agnatha vs Gnathostomotata

Placoderms: back to the Ordovician

2013: discovery in China -- the transition of placoderms to jawed fish greatly affected the evolution of sharks.

Placoderms: ur-jawed fish. 

Jawed fish: a class

Age of Fishes

Silurian (begins)
Devonian (huge)
Carboniferous (plays out)

The first jawed fish separated into four groups.

spiny sharks            placoderms            chondrickthyans            osteichthyans
(acanthodians)        (plated skins -        (cartilaginous fish)        (bony fish)
                                bone)

 

 Progression
    jawless --> jawed --> teeth (although teeth existed in jawless fish)
    respiratory relationship

400 mya: lots of water; warm climate / global warming --> jawed fish took off --> 200 genera

Once jawed fish arrived, only the jawless fish lampreys and hagfish survived.

Placoderms -- 400 mya but extinct by 359 mya -- obviously bony plates did not work; probably not a bit agile; by end of Devonian completely extinct when fish were flourishing

Acanthodians: survived until about 250 mya, later Permian;
    small spiny sharks (not true sharks)
        features of both sharks and bony fish
        Climatius -- huge advantageous adaptation
            an active hunter

Placoderm:
    Dunkleosteus -- top predator of the late Devonian -- and then disappeared

SHARK -- DEVONIAN
ORIGINS UNCLEAR

Absolutely fascinating: everything else went with bones; exception, sharks (and a few cousins)

chondrichthyans

    sharks, rays, skates, chimaeras (ratfish)

cartilage / bony teeth / dermal denticles

*** 2013: early type of placoderm shed light on evolution of sharks.

Thesis: bony fish may have retained ancient structure
    sharks may have been the ultimate innovators
    possible; bony skeletons came first
    then, cartilage skeletons evolved from bones!!
    rather than the other way around which was long assumed!

*** Shark evolution: a triumph of simple design -- and survived many mass extinctions; survived every mass extinction to which they were exposed.

    Sharks swam with placoderms and acanthodians but sharks outlasted them all.

Sharks: issue of sinking (p. 191)
    livers with lighter-than-water oils; and,
    large pectoral fins, buoyant (similar to birds)

LATE DEVONIAN MASS EXTINCTION

killed off placoderms, acanthodians, and most jawless fish

CARBONIFEROUS: bony and cartilaginous fish 

Reminder: chondrichthytes -- a CLASS of jawed fish.

Four classes of jawed fish; two classes extinct; two classes extant.

Sharks: 45 families (today, 40 families0

This period also saw emergence of the HOLOCEPHALI --
    another surviving group of Chondrichthyes

Subgroups:
Holocephali
    chimaeras, rabbit fish, elephant fish
Elasmobranchi
    sharks, rays, skates

Carboniferous extinction: rainforest collapse
    did not affect sharks

PERMIAN EXTINCTION
The Great Dying or the Permian-Triassic Extinction

252 mya: wiped out 90% of marine / 70% of land vertebrates.

200 mya: the sharks that survived the Permian extinction -- did very well -- great wave of evolution -- also produced true Batoids -- flattenend skates / rays - "wings"

100 mya: most moder shark groups had appeared

Cretaceous Extinction
This was the Cretaceous - Triassic
The Meteor

Again, the sharks survived and their cousins survived.

50 mya -- Hammerhead sharks appeared

    -- also the baleen whale and
    -- magamouth sharks
    -- manta rays

 

Well into the Cenozoic Era

Big mouth sharks
    great white shark -- contemporaries with the
    megalodon 

Megalodon last 14 million years
    two million years ago -- megalodon died out
    still around: great white shark

Ends with the Sixgill Shark -- cow sharks -- deep water
    

*******************************
Secrets of Sharks

Sharks:

• oceans' most feared predator, the white shark
• the oceans' weirdest-shape fish, the hammerhead
• the magnificent giant filter feeding whale shark and manta rays
• the voracious garbage eaters of the sea, the tiger shark
• the extraordinary bullhead sharks, docile little fished that are true living fossils, virtually unchanged from when dinosaurs ruled the land 150 mya

Their capabilities set them apart from all other fishes

• due to their long evolution: spanning some 465 million year

Other

• superb sense of smell
• can detect faint electrical fields of other living creatures (p. 9)
• teeth, remarkable; every species of shark has unique teeth

Origin of sharks: one of the last great unsolved mysteries in the 500-million-year-old evolution of backboned animals (vertebrates).

• did sharks evolve from first jaws and teeth or some other ancestral archaic fish group before them
• unlike most other vertebrates, no transitional forms found

Early on, p. 11: homeobox genes

paleontology: definition, p. 12

long periods of time, tens or hundreds of millions of years as deep time.

biostratifgraphy: use of fossils to date rocks within a narrow time range (p. 13)
within one or two million years
fossil sharks' teeth are often found as microfossils, useful for dating

*******************
Chapter Two

Arandaspis: the first early jawless fish. Oldest known fossil shark scales were found in the same layers of rock. So, a shark a contemporary of a jawless fish. 

Ordovician.

465 mya -- let's call it 500 mya -- half a billion years ago.

Australia part of Gondwana (southern continents). Laurentia (North Amerian) 3,000 miles away to the north.

Northern boundaries of Gondwana straddled the equator; nestled against what would be northern China. 

Global sea temperatures sweltering around 100°F at the equator -- boiling -- high seas -- between 380 and 680 feet above today's levels.

Larapintine Sea: shallow marine seaway the traversed Australia at the time.

Nautiloid, trilobites.

Jawless fish. Metasprigginia. Also found at the famous Burgess Shale in British Columbia, lived around 508 mya.

Diverse number of protovertebrates called conodonts.  Jaw-like mouthparts.

Cartilage notochord supporting their body rather than a backbone.

The biggest was Arandaspis. 

Also, Iowagnathus. Jawed predators hunting jawless prey.

The warm inland Larapintine Sea was home to the world's first shark, named Tantalepsis (p. 25). 

A nice time to review the continents at this time:

• pre-Cambrian: one large continent, Rodinia
• Paleozoic: initially breaks into two main continents (Gondwana and Laurasia)
• then further changes, greatly affecting the flora, fauna, and evolution of the age of fishes, from the Ordovician to the Silurian to the Devonian.
• the Larapintine Sea appeared during this period and was a major site of fish evolution

 

The Vendian is the period before the Cambian. The Vendian Period is divided into two epochs, the Varanger and the Ediacara. The Ediacaran epoch was marked by the appearance of the "Vendian biota" or "Ediacara fauna," a group of large soft-bodied organisms. 

Note:

Links:

• although this is about the Baltic Basin, the summary is very useful.

Middle Ordovician:

The Permian -- Pangaea (the two "P's" -- Permian and Pangaea; or three "P's" -- Permian -- Pangaea -- Perish) -- way too big; huge deserts:

*****************************
Sharks

 

NOW, back to sharks.

Page 26: sharks for the beginners. 

Page 34: Tantaleopsis: first shark?

• central Australia, south of Alice Springs; 2006
• placoid scales
• Tantalepsis: likely an early chondricthyan (cartilaginous fish) but not sure
• similar scales found in a group of ancient jawless fishes, called thelodonts
• tubular to flattened fishes
• but there is a difference between the scales of the thelodonts and sharks, including Tantalepsis
• based on this information from scales, Tantalepsis is as close as we can get to the oldest evidence for the origins of the shark group, the chondrichthyans (cartilaginous fish).
• the scales are very similar to the scales found on the nurse shark. Nurse sharks may be among the oldest sharks? Bottom feeders.

Time line of sharks. Very, very good site. Smithsonian.

Shark scales, p. 38.

First major mass extinction! End of Ordovician. Second worst mass extinction.

Remember, the period before the Ordovician was the Permian. The Permian explosion (of life). 

But all life on earth was marine so if there was going to a mass extinction of note, it would be marine life.

And that's exactly what it was. p. 39+. Great discussion.

The Ordovidian mass extinction, very clearly marked between 445 and 443 mya. Remember, the first sharks about 450 mya.

The shark that survived best represented by Solinalepsis, in the group of sharks with the most advanced scale structure, called the mogolepid  sharks. Very complex crowns made of a denser noncellular dentine with neck canals present -- a feature seen in the scales of living sharks.

Most sharks, shallow water, near the coasts, would have been knocked sideways by the declining water levels, but the author believes that there were sharks in deep ocean water that survived and were unaffected by the Ordovician mass extinction.

So, some sharks survived the Ordovician mass extinction.

Progress in the short but dramatic Silurian period.

Silurian Sharks

Fossil evidence:

• rising diversity of sharks are known from scales, as wells as the first fossil records of:
• isolated fin spines, and
• one nearly complete fossil shark

Sinacanthids.

Acanthodian-like stem sharks.

Mongolepid scales.

Oldest early shark known from an almost complete body fossil is Shenacanthus from Chongqing, South China, which lived around 437 mya.

Shenacanthus might bridge the gap between sharks and another early jawed fish called placoderms. Placoderms appear in the Early SIiurian at exactly the same time. 

Recap of the first 46 million years of shark history: bottom of page 42.

See graphic page 43: Tantalepis (Ordovician); Shenacanthus (Silurian) -- both branch off from main line leading to modern sharks.

Chapter 3: Sharks Become Predators
Sharks' First Superpower: Deadly Teeth

Searching fo the oldest known shark teeth

In Spain.

Early Devonian.

• oxygen levels about 8% higher than the present
• oxygen supercharged the metabolism of the first jawed vertebrates, which in trun drove an explosioin of fish diversity
• bizarre fish appeared and more diverse sharks appeared
• 415 - 408 mya
• a site where the oldest known sharks' teeth are found
• author's understanding of early sharks increased exponentially
• sharks' teeth are enlarged mirror images of their tiny placoid scales -- p. 47
• placoid scales around the mouth --> teeth --> first seen int eh Devonian -- p. 47
• how teeth developed may have been linked to their taste buds
• the same embryoni stem cells form both the teeth and the taste buds
• the regulator gene: Sox2
• see theory -- p. 48
• sharks' first real superpower: ability to replace teeth -- p. 50
• replacing teeth in a matter of days

Now, describing the Devonian (419 - 393 mya).

• throughout first 15 million years, sharks underwent radical changes
• prior to this:
• protosharks known only from scales, or acanthodians (stem sharks)
• all of a sudden paleontologist found sharks with teeth and replacing teeth like modern sharks
• again, early Devonian sharks
• until 2000, no complete shark fossil from this period had been found
• then, in 2000, serendipity -- not one, but two complete shark fossils found, in Canada

Author visits the Canadian site in 2014 -- p. 53.

 

Sharks' second superpower: smell.

Antarctica: big shifts in evolution.

Three big shifts:
larger
freshwater for the first time
higher diversity: at least four species living together in same ecosystem; fish were becoming very diverse and plentiful, allowing different species of sharks to feast on different species of other fish

Chapter 4: The First Rise of Sharks -- Sharks Lash Out

Late Devonian: 383 - 359 mya.

So the Devonian is very long: an early Devonian and a late Devonian. Plenty of time for sharks to get a real toehold.

Late Devonian: huge change for earth -- 24 million dramatic years, especially on land
vegetation changed dramatically
middle Devonian forests had low to midlevel canopies, while in the
late Devonian: the world's first towering forests made of gigantic plants
first tetrapods, early amphibians appeared
the seas were teeming with plankton; more free-swimming than floating forms
sharks flourish; mostly the same as before but one shark group rapidly increased in size

 

Late Devonian: recurrent events that shook up the earth

• events that choked the seas of oxygen
• not one but two rather minor extinctions
• p. 68
• the first: devastated life in the seas
• the second: culled all life in saltwater and freshwater

Finding a new Devonian shark 

again, back to Australia
the author's discovery, p. 70

Then Morocco

 

Then Southeast Asia, the Golden Triangle, p. 78

 

SHARKS REINVENT THEMSELVES, FOR THE FIRST TIME -- page 85.

Page 85: sharks reinvent themselves for the first time (they reinvent themselves twice; the first time was in the late Devonian: the first known holocephalan -- the group containing chimaerids, ratfishes, and spookfishes. These fossils are generally found in younger Carboniferous rocks.

Chapter 5: Sharks' Armored Rival -- Sharks Versus Placoderms
Dunkelosteous -- dark lord of the Devonian deep and the shark's mortal enemy -- p. 88

Ancient American sea, 359 mya

 Why the placoderms were such a threat to the sharks.

The dunkelosteous and the Gorgonichthys.

Ohio -- p. 90

 

Page 105: tree of life -- bony fishes + tetrapods (US)

Trunk: Placoderms (jawed-skull made of bony plates) -- main line leads to early ray-finned fish; breaking off from main line:
"conventional" placoderm
maxillate placoderms
main line: bony skull retained and refined; breaking off from this main line:
bony skull lost: toward SHARKS (toward sharks, rays and ratfishes)
lobe-finned fishes, the main line of this branch leads to tetrapods, mammals and humans
twigs branching off from this branch: early lobe-finned fish; living lobe-finned fish
main line: still trending toward early ray-finned fish, and living ray-finned fish

So: main line from placoderms to ray-finned fish
branching off:
sharks first
later, lobe-finned fish --> tetrapods

finale: Devonian sharks triumph over placoderms
placoderms: bony plates, probably less maneuverable; large energy requirements for weight
sharks: stream-lines; cartilage, less heavy than bone

See tree of life: Cambrian - Ordovician - Silurian (brief) - Devonian (long and impactful) -- p. 116.

Part 2
Sharks Rule

Chapter 6: The First Golden Age of Sharks -- Sharks Take Over The World

An ancient Montana bay, 318 mya
describes the supercontinents at this time, bottom of page 121

This is amazing: like the dinosaur finds in Montana, the same for sharks in Montana

p. 146: Carboniferousshark wrap-up.

sharks and their kin, the holocephalans, diversified as never before in the first few million years of the Carboniferous period; 100's of new species, representing both predatory and shell-crushing species, suddenly appeared.

If the Cambrian was the "Big Bang" for life on earth, the Carboniferous was the "Big Bang" for sharks.

Bear Gulch, Montana. New York Times, 1976.
Bear Gulch, fossil hunting, 2007.

Chapter 7: Swamp Sharks -- Sharks Take Over Rivers And Swamps

Chapter 8: Rise of the Buzz-Saw Sharks -- How Sharks With Wheels of Teeth Dominated the Oceans

 

Part 3
Sharks Under Pressure

Chapter 9: Sharks And The Great Dying -- Earth's Biggest Extinction Event Shapes Shark Evolution

The chapter begins with a recap.

Chapter 10: The Jurassic Rise of Modern Sharks -- Little Sharks Hold Their Own

Chapter 11: Sharks Go Large -- Sharks Versus Giant Marine Reptiles

Part 4
The Age of the Megasharks

Chapter 12: Sharks After The Impact -- The Rise of Modern Sharks

Chapter 13: Ascent of the Superpredators -- How Shark Predators Got Very Large Very Quickly

Chapter 14: Megalodon -- The Greatest Superpredator Ever

Part 5
Sharks Today

Chapter 15: White Shark -- A Natural and Cultural History of an Iconic Living Shark

Great tree of life, p. 365.

 

Chapter 16: Sharks and Humans -- Can Sharks and Humans Live Together?

Epilogue: The Wisdom of Sharks

Acknowledgments

Notes on Sources

Glossary of Terms

Index, p. 449

******************************
An Extinction Nineteen MYA

 

a

 

 

********************
Whales

Why they appeared so late.

 

*******************************
Montana: Paleontology

In the United States, in Montana:

If you want to study dinosaurs, you go to Hell Creek, eastern Montana.

If you want to study the evolution of fish / sharks, you go to Bear Gulch, Fergus County, near Lewistown, Montana.

The Bear Gulch Limestone is commonly considered to be part of the Heath Formation, the youngest formation in the Big Snowy Group of central Montana. 

Some authors instead consider the Bear Gulch Limestone to be an early member of the Tyler Formation, a patchy but widespread unit of Carboniferous limestone and terrestrial sediments.

Most of the Heath Formation is represented by black shales and marls, indicative of brackish and salty littoral environments. It developed along a transgressive sequence in a narrow saltwater seaway, known as the Central Montana Trough  or Big Snowy Trough. This seaway flowed into the Williston Basin, a shallow inland sea further east. The Central Montana Trough would have also been linked to fully marine basins on the western coastline of Laurussia, but this connection may have been broken by the time of the Bear Gulch Limestone's deposition.

Many distinct limestone lenses (localized sediment packages) are developed in the Heath Formation. They overlap each other in an east-to-west sequence which extends over a distance of 160 km in the Central Montana Trough. The only exposed portions of the sequence are found at Potter Creek Dome, a small uplifted area northeast of the Big Snowy Mountains. The last few limestone lenses form a large portion of the Upper Heath Formation, which is sometimes termed the Bear Gulch Member in recognition of the most well-exposed and fossiliferous lens in the sequence. This lens, the Bear Gulch Limestone, was also one of the last in the sequence, only succeeded by the Surenough Beds immediately west of it.  The Bear Gulch Limestone can be observed in numerous outcrops, spread out over an area of more than 20 square miles in Fergus County, Montana.

The creation of limestone lenses in the Heath Formation has been linked to tectonic activity extending the seaway by excavating bays out of the surrounding land. As old bays are filled in and buried by sediment, faulting and seismic events form new bays in a long eastward to westward succession.  It may have taken a mere 1000 years for the bay responsible for the Bear Gulch Limestone lens to fill in completely, after only 25,000 years for the entire bay formation sequence to run its course across Montana. 

The final limestone deposits in the area were succeeded by freshwater lake sediments of the Cameron Creek Formation, the oldest unit of the early Pennsylvanian-age Amsden Group.