Chapter 6 – Basic Concepts in Dinosaur Taphonomy
Introduction
Definition and Importance of
Taphonomy
The Study of everything that happens
to an organism’s body after it dies
It relates to:
Probable cause of death
Evidence of scavenging
Bloating
Time before burial
Dismemberment of the carcass
Transport from the death site
Changes after burial
Why animals become fossils and others don’t
Completeness of fossil record
Fossil record very incomplete,
particularly for terrestrial animals like dinosaurs
How representative is the biased fossil record of the original
dinosaur population?
50% of dinosaur species known from only one specimen, sometimes only a partial skeleton
80% of dinosaur species known from less than five specimens
How do deposits of carnivores only originate?
Terms
Intraspecific – within
the same species
Interspecific – between
different species
In situ
– in place
Trace fossils are always in situ, many body fossils likely were moved
Possible Causes of Injury, Poor
Health and Death in Dinosaurs
See List on page 123
Monospecific – single
species
Paleopathology – the
study of sickness, injuries
and other abnormalities in the health of ancient organisms
Dinosaurs mostly very healthy, except for a few individuals, particularly theropods
Sedimentary Environments and
Dinosaur Preservation
Continental (nonmarine) environments
Fluvial - meandering rivers & braided streams
Lacustrine - lakes
Desert - sand dunes (eolian), playa lakes, alluvial fans
Transitional (shoreline) environments
Deltas - river mouths
Estuaries – coastal embayments where fresh and salt water mixes that form where tidal ranges are large (>2 m)
Barrier islands - form where tidal ranges are small (<1 m)
Marine environments
Shallow-marine – continental shelves – dinosaur fossil very rarely found
Sedimentary rock classification
Composition, texture
Clastic sedimentary rocks
Shale, mudstone, siltstone, sandstone, graywacke, conglomerate, breccia
Chemical sedimentary rocks
Limestone, dolostone, chert, hematite, gypsum, halite, coal
Facies
All the characteristics imparted by an environment to its sediment at
the time of and shortly after deposition
Lithofacies, biofacies, ichnofacies
Postmortem Processes (Pre-Burial)
Definition of Biostratinomy
Study of the complicated series of post-death
processes that involved both biological and physical factors
Biological Processes: Decay and
Scavenging
Necrolysis – decomposition of
a body after it dies
Rigor mortis bends body into arc
Anaerobic and aerobic bacteria start
decomposition that gives off gases
Carcass bloats and eventually explodes
Gas emissions attract scavenging
animals
Insects, other arthropods in aqueous environments, theropods, pterosaurs, reptiles, mammals, birds
Soft tissue stripped, leaving
bones
Bones attacked by beetles and other organisms
Four to six weeks, nothing
left
“Bloat and Float”
Scattering of body parts by scavengers
Physical
Processes: Water and Wind
Autochthonous vs Allochthonous
In place vs transported
Transport processes in water or
wind
Traction, Saltation, Suspension
Can determine transport parameters in
ancient sediments from distinctive bedforms like ripples or dunes
Transport of dinosaur carcasses or body parts dependent on velocity of wind and water and on stage of decay of dinosaur carcass, which affects its density (0.9 to 1.1 grams per cubic centimeter)
Recently dead will drag and bounce along a relatively shallow bottom
(traction and saltation)
A bloated carcass will float a considerable distance (suspension)
Feathered dinosaurs would also float long distances because of the
buoyancy provided by air trapped in the feathers and their hollow bones
“Dinosaur jams” if many bodies in the water
Transport of dinosaur bones depends on whether they are dominantly cancellous or compact and their shape (See Table 6.3)
Compact bone AND teeth denser than water and moved by traction on
bottom
Abrasion and fracturing
Cancellous bone less dense than compact bone and teeth
Shape affects lift by varying the surface area relative to the volume - Flat or long particles can be lifted
more easily than spherical particles
Useful to map orientation of dinosaur bones, as done for the Howe Quarry, which demonstrated that this Upper Jurassic deposit could have formed as a crevasse splay
Postmortem Processes: Accumulation,
Burial and Post-Burial
Accumulation and Burial
Why dinosaur bones accumulate
The assemblage is authochthonous
Transport of allochthonous
bones/carcasses ceases
Bones are reworked
Bone beds
Lots of bones of many individuals of
only a few species
If a bone bed is authochthonous, it
may resemble the life assemblage (Biocoenosis)
Co-occurrence of predator and prey in a small area have been interpreted as actual interactions between the species
This clearly seems the case for “The Fighting Dinosaurs,”
where a Velociraptor and Protoceratops are interacting (see Figure
6.9)
Probable for several Deinonychus and Tenontosaurus as well
Autochthonous deposits may accumulate from sediment traps (Allosaurus at Cleveland-Lloyd), drought-related
mortality (Parasaurolophus and
Styracosaurus in a late Cretaceous example), volcanic ashfalls (Hypacrosaurus in Montana), or
fissure accumulations (Iguanodon
at Bernissart and Plateosaurus in
Germany)
Relatively rapid burial is required, probably within 4 to 6 weeks
after death
River flooding (Coelophysis
at Ghost Ranch) and desert sandstorms or collapse of wet sand from a sand
dune (Oviraptor on its
nest and “The Fighting Dinosaurs,” both from the Gobi) are
processes that would do this
Slow burial under anaerobic conditions like at the bottom of a lake
would also do
If a bone bed is allochthonous, it comprises a death assemblage (Thanatocoensis)
Interpretation of a thanatocoensis in ecological terms is tricky
Organisms associated in a thanatocoensis may not have lived together, either in time or space
Post-Burial Processes: diagenesis and
How Bones Stayed Preserved AND Preservation of Dinosaur Skin Impressions and
Soft-Part Anatomy
Diagenesis
Biological, Chemical and Physical Processes
that occur beneath the surface that are capable of changing an organisms
remains (or the sediment or rock itself)
A
dinosaur body would alter the character of the surrounding sediment
Decay of soft tissue would generate gases that could change the geochemistry of the surrounding sediment
Ground water tends to be acidic and contain
dissolved trace elements
A pH more
acidic than 5 will dissolve bones and teeth
Fossilization processes
Unaltered body fossils
Bone &teeth minerals (dahllite) remain unchanged, a process that is extremely rare
Altered body fossils
Permineralization: porous bones filled with secondary
minerals
Can
concentrate trace elements and make bones radioactive
Replacement: dissolution of original bone or tooth
minerals, precipitation
of new mineral, equal in
volume to that dissolved
Pseudomorphing
is case of replacement that can preserve the finest details
Recrystallization: dissolution & reprecipitation of bone or tooth minerals
These can
occur rapidly – anywhere
from a few hours to a few years and are probably necessary for long term
preservation of bones and teeth and can “seal” in biomolecules
Carbonization: soft tissues preserved as thin carbon films
Molds (dissolution of bone) & Casts (filling of mold)
Mummification