The “Black Sheep” of the Ichthyosaur Family

Making Heads or Tails of Sclerocormus parviceps

No sooner has the likes of Oliver Rieppel, (Rowe Family Curator of Evolutionary Biology at the Field Museum, Chicago) and Nick Fraser (National Museums of Scotland), in collaboration with scientists from China “unzipped” the bizarre Triassic marine reptile Atopodentatus unicus*, another academic paper comes along introducing the equally peculiar looking Sclerocormus parviceps to the world.  A number of media outlets have already covered this news story, but in this article we will take a slightly different approach, focusing on the rapid evolutionary changes that seem to have taken place in marine ecosystems following the End Permian mass extinction event.

The Complete Specimen of Sclerocormus parviceps (Holotype Reference AGB6265)

Sclerocormus parviceps fossil material.

Sclerocormus parviceps fossil material.

Picture Credit: Da-yong Jiang (Peking University)

The photograph above shows the various components that make up the holotype specimen.  The scale bars to the left and in the centre each  measure 25 centimetres in length.  The long, thin, whip-like tail is on the left of the photograph, the skull on the right.

The specimen was excavated from Bed 719 in the Majiashan Quarry, near the city of Chaohu in Anhui Province (eastern China).  These, predominately limestone beds are highly fossiliferous and a huge variety of invertebrate and vertebrate fossils are known from this location.  The strata was laid down in a shallow, tropical sea environment not long after the End Permian extinction event and as a result, the fossils found in these rocks have provided palaeontologists with a unique opportunity to examine how marine fauna bounced back after the extinction that is believed to have wiped out around 95% of all marine life.

Sclerocormus parviceps – Not Your Typical Ichthyosauriform

Writing in the on line journal “Scientific Reports”, the researchers describe this new type of marine reptile as a basal member of the ichthyosauriforms.  The fossil material dates from around 248 million years ago (Olenekian faunal stage of the Lower Triassic), it suggests that if a long-tailed, short-snouted marine reptile existed at this time, then these reptiles must have diversified rapidly during the Early Triassic.  Although, related to the Ichthyosaurs, those dolphin-like, super sleek animals with their long jaws and fishy tails, as S. parviceps was so different from the typical Ichthyosaur body plan it indicates that there must have been a period of very rapid evolution.

A Scale Drawing of Sclerocormus parviceps

A scale drawing of Sclerocormus parviceps.

A scale drawing of Sclerocormus parviceps.

Picture Credit: Everything Dinosaur modified from an illustration by Nicolay Zverkov

Measuring around 1.6 metres long from that stubby snout to the end of the serpentine tail Sclerocormus parviceps is one of the largest types of marine reptile known from the Lower Triassic.  Based on an analysis of the fossil material, in comparison to Late Triassic Ichthyosaurs, Sclerocormus would have been a relatively poor swimmer.  In addition, whilst many of the later Ichthyosaurs evolved into apex predators, the short -snouted Sclerocormus seems to have specialised in eating small, soft-bodied creatures.  The scientists speculate that the toothless Sclerocormus used its short snout to create pressure and suck up food like a syringe.  It looks very different from its relatives and it probably behaved very differently too.  Sclerocormus could be considered the “black-sheep” of this particular group of marine reptiles and as it was so very different from its relatives it tells palaeontologists a lot about the rapid exploitation of ecosystem niches by vertebrates after the mass extinction event.

The Skull of Sclerocormus parviceps (b) and a Line Drawing Illustrating the Individual Bones (b2)

The skull of Sclerocormus parviceps (b) with an explanatory line drawing identifying individual bones (b2).

The skull of Sclerocormus parviceps (b) with an explanatory line drawing identifying individual bones (b2).

Picture Credit: Scientific Reports

The picture above shows (b) a close up of the skull material and (b2) a line drawing indicating the individual position of the bones.  Although, crushed the palaeontologists can make out a lot of detail regarding the skull morphology of this marine reptile.  The scale bars represent 1 cm (b) and 2 cm (b2).

Key

Skull elements. Abbreviations: a?, angular; ar?, articular; d?, dentary; f, frontal; j?, jugal, l?, lacrimal; m, maxilla; n, nasal; p, parietal; pm, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; q, quadrate; sa?, surangular; scl, scleral ossicles; sq, squamosal and st, supratemporal.

Commenting on the significance of this discovery, Dr. Rieppel stated:

“Sclerocormus tells us that ichthyosauriforms evolved and diversified rapidly at the end of the Lower Triassic period.  We don’t have many marine reptile fossils from this period, so this specimen is important because it suggests that there’s diversity that hasn’t been uncovered yet.”

The Triassic Timescale and Marine Reptile Diversity

Although, the marine reptile fossil record for the Lower and Middle Triassic is poor, scientists have been able to use recent fossil discoveries from China to help plot the evolution of this important group of marine reptiles.  The authors of the scientific paper plotted the diversity of species during the Triassic and their evidence suggests that there was not a single burst of evolution during the Early to the Middle Triassic, but at least two waves of marine reptile diversification, separated by a decline in speciation.  In addition, the scientists conclude that the first burst of evolution led to a rapid increase in the Ichthyosauromorpha, the ichthyosauriforms such as  Sclerocormus and the related Hupehsuchia.  This was followed by a second period of rapid marine evolution, but this time it was the Sauropterygia (Placodonts and their relatives) and the closely related Saurosphargidae that seem to have evolved most rapidly.

Two Bursts of Marine Reptile Evolution in the Early to Mid Triassic

The diversity of Triassic marine reptiles.

The diversity of Triassic marine reptiles.

Picture Credit: Scientific Reports with additional annotation by Everything Dinosaur

The graph above shows species diversity over time through the Triassic.  The black line in the graph shows raw data from the fossil record, however, as the fossil record for marine reptiles from this time is far from complete, the scientists have inflated the stratigraphic ranges of species to account for gaps in the fossil record.  The red dotted line plots the data where half of the species are assumed to have their records missing from the faunal sub-stages of the Triassic.  The blue dotted line shows a more extreme view where all species known to date are assumed to be missing records from the faunal sub-stages before and after the actual date of a fossil discovery.  The data, however it is plotted, does show two distinct evolutionary phases, lots of new species in a relatively short time.

Explaining the team’s findings in the context of Darwinian evolution, Dr. Rieppel explained:

“Darwin’s model of evolution consists of small, gradual changes over a long period of time, and that’s not quite what we’re seeing here.  These ichthyosauriforms seem to have evolved very quickly, in short bursts of lots of change, in leaps and bounds.”

Gradual change, a sort of slow, creeping evolution may not have taken place in shallow seas immediately following the End Permian extinction event.  With so many vacant niches to exploit, those vertebrates that survived the extinction seem to have radiated rapidly, evolving a myriad of specialist forms such as Sclerocormus parviceps.  This may be an example of “punctuated equilibrium” a form of evolution proposed by the famous palaeontologist Stephen Jay Gould and others whereby evolutionary change occurs relatively rapidly, alternating with longer periods of relative evolutionary stability.

The Triassic Timescale – An Explanation

In the line graph above, Everything Dinosaur team members have labelled the three Epochs that form the Triassic on the horizontal timeline (Lower, Middle and Upper).  Underneath the graph, we have annotated the faunal sub-stages listed by providing a key showing the faunal stage that each sub-stage is associated with (Olenekian, Anisian, Ladinian, Carnian and Norian).  For some readers the term faunal sub-stage may be unfamiliar to them, here is a brief explanation.

Biostratigraphic in conjunction with relative dating and more recently methods such as radiometric dating and palaeomagnetism have enabled scientists to date events preserved in the geological record.  These time intervals allow a geological period, in this case the Triassic to be established.  However, to explore in more detail the geological record laid down over the tens of millions of years represented by a geological period, this immense amount of time is further divided up into epochs, faunal stages and sub-faunal stages.  It is just like the pages of a book being divided up into paragraphs, the pages themselves being grouped into chapters.

The Triassic comprises of three Epochs (also called series): Lower, Middle and Upper

These in turn are divided into seven faunal stages:
Lower = Induan, Olenekian
Middle = Anisian, Ladinian
Upper = Carnian, Norian, Rhaetian

As the faunal stages themselves represent many millions of years, changes in the geological record are used to divide these stages themselves into a series of smaller units called faunal sub-stages:

The seven faunal stages of the Triassic are further divided into fifteen faunal sub-stages namely:

Induan = upper Griesbachian and the Dienerian

Olenekian = Smithian and the Spathian

Anisian= Aegean, Bithynian, Pelsonian and the Illyrian

Ladinian =Fassanian, Longobardian

Carnian = Julian and the Tuvalian

Norian = Lacian, Alaunian, Sevatian

*To read an update on the bizarre Triassic marine reptile Atopodentatus unicus: Atopodentatus unzipped

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