Most of us think of fossil vertebrate remains as being
bones or teeth. In rare cases we can sometimes see skin
impressions or the outline of feathers on fossil birds and
dinosaurs, but these are among the top 1% for fossil preser-
vation.
Such exquisite fossils come from deposits called
konservat laggerstätten(meaning “place of storage”), which
are the result of rapid burial and special chemical condi-
tions involving low-oxygen environments. In special cases
these deposits can preserve or mineralogically “ghost” a
range of soft tissues, even complete internal organs, in
the fossilised organism.
Scientists have long known about the famous Burgess
Shale fossils in British Columbia, where soft-bodied
worms and other invertebrate creatures were buried by
rapid mud slides around 508 million years ago. However,
well-preserved ishes from the 113–119 million-year-old
Santana Formation of Brazil were among the irst verte-
brate fossils to show evidence of preserved soft tissues.
These include parts of the glandular stomachs and bands
of muscles, with the original tissue mostly replaced by
chemical processes.
To ind whole preserved internal organs in a fossil is the
Holy Grail of palaeontology. Such discoveries contribute a
wealth of new anatomical information that is essential for
understanding evolutionary patterns.
Therefore the recent discovery of a complete, well-preserved
fossilised heart in an almost 120 million year old ish
(https://elifesciences.org/content/5/e14698) was a major
breakthrough for the team of largely Brazilian scientists led by
José Xavier-Neto of the Brazilian Biosciences National Labo-
ratory. The heart is perfectly preserved in 3D in a number of
specimens, even showing valves that regulate the outlow of
blood from the conus arteriosus. The Rhacolepisspecimens ill
in a gap from what is known from primitive living ray-inned
ishes with many heart valves to the teleosts, which have only
one valve.
This new inding used synchrotron X-ray tomography to
image Rhacolepisfossils that were still entombed within lime-
stone concretions. This technique images the fossil in thin
sections; these images can then be processed to render the heart
slice by slice and digitally restore the features of the organ. This
method has been widely applied in palaeontology for the past
decade or so to reveal many intricate soft tissue structures in
fossils.
Other examples of superb soft-tissue preservation in fossils
include the remarkable preservation of actual fossilised sperm
in a 16 million-year-old ostracode (a bivalved crustacean) from
the World Heritage Riversleigh sites in Queensland (Fossil File,
July 2014), and a preserved shrunken brain inside the braincase
of a 300 million-year-old shark-like ish from North America.
Our 380 million-year-old Gogo ish fossils from Western
Australia have yielded a collection of superbly preserved soft
tissues that include the twisted umbilical cords associated with
an embryo, nerve axial plate cells and muscle bundles. Recently
the complete neural and vascular systems were reported in 520
million-year-old arthropods from Chengjiang in China,
preserved as differently coloured chemical traces of the orig-
inal brain, nerve cords, blood vessels and veins.
However, caution must always be exercised when identi-
fying such structures in fossils. About 16 years ago a fossilised
heart made international news headlines when it was reported
in a Cretaceous plant-eating dinosaur (Thescelosaurus) nick-
named Willo. A large scale CT scan of the dinosaur’s body
cavity showed a heart-like structure preserved in ironstone. In
2011 a new study demonstrated that it is not a heart but the inill
of iron-rich sediment washed into the decaying body cavity of
the animal.JUNE 2016|| 43John Long is Strategic Professor in Palaeontology at Flinders University, and current
President of the Society of Vertebrate Paleontology.THE FOSSIL FILE John Long
The Heart of a Good Fossil
Palaeontologists have found their Holy Grail: the fossilied heart of a Cretacean fish.
Top: the fossilised heart preserved in the 120 million-year-old fish
Rhacolepisfrom Brazil. Below: white arrows show the position of
valves. Credit: Maldanis et al 2016, eLife