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The Extinctions

 

The extinction event/s affected life on both land and at sea.  The number of families decreased by around 23% in both environments.

Chart of species range across the Triassic Jurassic boundary.

Click to enlarge

Picture taken from the IDEO website click here to go to the website.

 

Marine life

 

Cephalopods

Early on during the mid-Triassic stage (Smithion), the ammonite species Xenodiscaceae became extinct and Noritaceae and Dinoritaceae's populations suffered.

Picture of an ammonite.

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Chart showing the abundance of the ammonoid orders and superfamilies through time.

Click to Enlarge (WARNING large file, may take up to 45 seconds)

(from House 1988)

 

By the end of the Triassic, 58 cephalopod families disappeared, including all of the 46 late Triassic ceratitid cephalopods families.  6 ammonite families had too became extinct.

What are Cephalopods?

Cephalopods are a group of marine organisms which include, squids, octopus, cuttlefish and nautilus, which are still alive today, but also includes important groups from the geological past, including ammonites,  and belemnites (cuttlefish like organisms).

They are characterised by having more than 8 tentacles arranged around the mouth.  The group ranges from the Cambrian the the present.  The group however, hits it's maximum diversity in the Mesozoic. (see timeline).

Conodonts

Conodonts were abundant during the Smithion but shortly after their populations were dramatically reduced at each late-Triassic stage.  By the end of the Triassic, they became completely extinct.

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Click to Enlarge.

Illustrated to the Left are the remains of the conodont animal which are most often found.  The structures are 'teeth', as this was all that is commonly preserved many bizarre reconstructions of the animal made until a whole specimen was discovered.

The illustration to the right, shows the reconstruction of the conodont animal according to Mark Purnell.

Both Pictures taken from Life an unauthorised biography, by R Fortey (1997)

What are Conodonts?

Conodonts are animals which are best known from teethlike fossils, see above picture.  The species is represented by evidence from the Cambrian to the Late Triassic.  They have since been described as a jawless (no hinge or muscles) type of 'eel' rather like a lamprey today (see above reconstruction).

Conodonts are found from the Upper Cambrian, to their final extinction at the end of the Triassic.

Ostracods

The Ostracods, Palaeocopida almost went extinct and Cytheracea and Bairdacea's population was markedly decreased.

Picture of a modern female Ostracod

Taken from a students text-book of Zoology, by A Sedgwick (1909)

 

What are Ostrocods?

Oscrocods are soft bodied organisms which are encased by a tough outer shell made of 2 valves (two halves upper and lower).  These animals are typically about 1mm in diameter.

The Ostracod group has been around for about 500 million years.  They arose in the Early Cambrian to Late Ordovician period, and are still abundant in just about every watery habitat on earth

Bivalves

In all 8 families were lost. 

Nearly half the bivalve genera disappeared at or in the run up to the T-J boundary.  The bivalves which lived in the deep neritic realm (a term for water of depth up to about 200m)  were most vulnerable to extinctions and at the end of the Norian the genus Halobia and Monotis disappeared from the fossil record.

Hallam estimated that 92% of NW European bivalves became extinct.

What are Bivalves?

Soft bodied organisms, enclosed within a 2 valved shell.  Oysters, cockles and muscles are living examples.  The 2 valves are usually mirror images of themselves.

 Bivlaves have adapted to many forms of life, fixed to the sediment or rocks; free swimming; free lying on the sediment; burrowing; boring into rock and wood, and are able to live in salt and freshwater.

The group has representatives from the Lower Cambrian to the Present.

Reef organisms

Scleractinians were abundant in the Norian but diversity decreased abruptly at the end-Triassic event/s.  Only 11/50 Scleractinian genera survived the late Triassic event/s.  The sponges, for example lost 8 families.

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Picture of Triassic Hexacorals (or sceractinian corals).

(Click to Enlarge)

Taken from Stanley (1993)

Brachiopods

12 families disappeared.  11 dielasmatid genera died, leaving just one species of that genus behind. 

Drawing of Brachiopods

 from Henry Woods Palaeontology Invertebrate (1963)

brach.jpg (38613 bytes)

Click to enlarge.

Chart of the Sucess and range of major Brachiopod groups.

From Invertebrate Palaeontology and Evolution By E.N.K Clarkson (1998)

 

Brachiopods

A marine invertebrate (has no internal skeleton).  They are sessile animals meaning that they are attached to the substrait usually my means of a fleshy 'ancour'.  The shells of brachiopods are bilaterally symetrical, i.e. the left side of the same valve is the mirror image of the right.

The group ranges from the Lower Cambrian too today.

Gastropods

13 families were lost, such as the murchisoniacian which became extinct at the T-J boundary. 

Gastropods

Well  known today as snails, whelk and cowreys.  The group is mainly aquatic found both in marine and freshwater environments, there are also terrestrial forms including slugs and snails.  Gastropods secrete a calcium rich shell with only one opening at the living end of the shell the other end is closed in a peak.

Gastropods range from the Cambrian to the present, they were the most abundant during the tertiary period (see Timeline)

Other marine life

Before the end Triassic event, at the end of the Carnian, 4 marine tetrapod families were lost, leaving only 2 families.  By the end of the Triassic 13 marine reptile families were lost, such as the Nothosauras and the Placodonts.

notho.jpg (28964 bytes) The fossilised remains of the nothosaur Ceresiosaurus. 

This specimen is from the Middle Triassic Muschelkalk of Germany and is displayed in the Museum für Geologie und Paläontologie, Türbingen, Germany) 

Picture from Exploring Earth and Life Through Time by S Stanley 1993

Terrestrial life

Pollen and spores

Pollen and spore counts suggest there were several peaks of extinctions of terrestrial plants at the end of the Carnian.  There is little strong palynomorphological evidence however for the Triassic Jurassic extinctions.

Tetrapods

13 diverse families became extinct at the end of the Carnian, such as the Rhynchosaurs and  Thecodonts.  6 further families became extinct at the end of the Triassic. 

Click to enlarge the picture of a Rhyncosaur on of the Tetrapod species that went extinct towards the close of the Triassic.

 

What are Tetrapods?

A tetrapod is any vertebrate (an animal with a backbone) which has 4 legs (could be a bird, reptile, amphibian or a mammal).

Plants

On approach to the T-J event/s, seed ferns such as the Glossopteridaceae, Peltaspermaceae and Corystospermaceea became extinct.

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A fossil fern from the Upper Triassic of New Mexico (U.S. Geological Service)

                                                (Click to Enlarge)

Picture from Exploring Earth and Life Through Time by S Stanley 1993

Other terrestial life

35 insect families, 8 freshwater bony fish families.

Vertebrates

There is great debate at present as to what Vertebrates went extinct and when.  Some think that the extinctions all occurred at the T-J boundary, whilst others think there were several extinction events affecting the vertebrates which occurred on approach to the T-J boundary.  More evidence will have to be collected before we know exactly what happened to the land vertebrates at the time.

The fossil record shows that there is no doubt that one or more extinction events occurred at, or on approach to the T-J boundary.  However until more conclusive evidence has been collected, it will remain unclear as to the exact nature of these extinctions.

For example, if evidence shows that a particular species was abundant at one time, and extinct a few million years later, we still do not know the nature of the extinction.  Perhaps there was a gradual decrease in population, or was there a catastrophic event in which huge populations were wiped out in one short event?  This feature is due to inaccuracy of the fossil record.  

Inaccuracy of the fossil record.

The fossil record is poor, as an organisms chances of fossilising are very remote. To be fossilised you have to end up in the right place at the right time, and have the right conditions.  You also have to not be eaten, scavenged, or deformed in the process of changing soft sediment into hard rock.  Only if all these conditions are met then will you be preserved.

As a result of these factors, the preservation potential of some organisms is very low, and some species are more than likely to have been lost without a trace.

Since only a very small percentage of a species are preserved, the chances of the youngest memeber of a species in an outcrop, being the last of it's species are astronomically small.  This means it is very hard to determine if an extinction event occurred, or if there was a gradual decline in the species, or even when the last one was alive.

We will not be sure of the cause of the extinctions until evidence is collected which fills these kind of time gaps to solve this problem. 

We are also restricted by the lack of knowledge on absolute timing.  There is still much dispute upon the exact dating for the sub-stages of the Triassic and of the T-J boundary date itself.

If we don’t know the precise dates, we cannot link the fossil record to other recorded events which may have been responsible for the extinctions.

 

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