Leedsichthys – Perhaps the Biggest Fish of all Time

Continuing the fishy theme from yesterdays post about the Coelacanth catch off the coast of Tanzania:-

Coelacanth article here: Coelacanth caught off Zanzibar

I thought I would write about another amazing fish, Leedsichthys, which was perhaps the biggest fish of all time.  The first remains of this huge animal were unearthed by the English farmer /country gent/ amateur palaeontologist and geologist, Alfred Leeds in 1887 as he explored some Jurassic sediments near Oxford, England.  He only found a few isolated fragments but other scientists on examining the preserved gill rakers of this animal estimated that it was a huge fish over 30 feet long.  The lack of fossilised skeleton parts including the spine (which palaeontologist think was made of cartilage and this does not fossilise well), and the incomplete remains meant that accurate estimates for this animal’s size could not be made.

The lack of fossils to study led scientists to name the animal Leedsichthys problematicus “problematical Leeds fish”.

An Illustration of Leedsichthys

Artwork © Ray Troll 1994

Further partial specimens of this giant Jurassic filter feeder came to light and up until recently the best Leedsichthys specimen was stored at Glasgow University’s Hunterian museum.  The most complete fossil was named “big Meg” and was donated to museum by Alfred Leeds in memory of his Scottish wife in 1915.  However, this animal, although reasonably well preserved was very fragmentary.  It had to be stored in 20 museum draws, so no really accurate measurements as to this animal’s true size could be made.

The drawing above was made by Ray Troll an American artist and illustrator who produces really cool pictures of fish and other creatures inspired by his studio in Alaska.

In the late 1980’s the existing fossils of Leedsichthys were once again re-examined.  The remains were compared to the a close relative of Leedsichthys called Asthenocormus. Using this technique of comparative anatomy new estimates of body size were made.  This publishes work indicated that Leedsichthys may well have reached lengths in excess of 30 metres (100 feet).  Doubts have been cast over the accuracy of these results.

A new much more complete Leedsichthys was found in a brick clay pit near Peterborough, Cambridgeshire (England) in 2001.  A team from Glasgow university were given the task of excavating the delicate remains.  It took a very long time, as there were lots and lots of fragments to recover from the Oxford Clay.  For example, it is estimated that Leedsichthys had something like 40,000 bristle like teeth in its huge mouth.

Despite the back bone not being preserved, scientists have estimated the animal’s size and put it at between 20 to 30 metres (65 – 100 feet) in length.  Bigger than today’s Whale Shark.  This makes Leedsichthys the biggest fish known to date.

It is thought that this animal cruised the shallow seas of the mid Jurassic, hoovering up small crustaceans, plankton and algae as it slowly swam.  It may have lived in shoals or swam around singly, only meeting up with others of its kind to find a mate.  Close examination of the remains of the 2001 specimen show growth rings on the bones.  It is believed that this animal was over 100 years old when it died and sank to the bottom of the sea.

As a genus the Leedsichthys were not around for very long, their remains have so far been found in sediments between 165 mya to 155 mya, from the Callovian to the late Kimmeridgian.  It is not known what caused the demise of these huge leviathans, perhaps they were not able to compete with the more advanced teleost fish that started to become more abundant at the end of the Jurassic.  Perhaps as the sea levels rose so Leedsichthys lost its shallow sea habitats – an early victim of global warming?

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