INTRODUCTION

The analytical stage of forensic anthropology involves answering questions that lead to identification of the individual whose remains are being examined.  The questions asked in developing a biological or demographic profile for an individual include the following:

• What is the race of the individual?
• What is the sex of the individual?
• What is the age of the individual?
• What is the stature of the individual?
• What traumas did the individual have?
• What pathologies did the individual have?
• What idiosyncratic traits did the individual have?

 
OBJECTIVES

• to use the hand lens for magnified examination of skeletal traumas

• to describe the effects of taphonomic processes on skeletal material

• to practice carefully handling skeletal material

READINGS

• Review the lecture notes on taphonomy.

• Review Chapter 16 in the Byers text.

• Review Chapter 16 in the Byers and Myster lab manual.
  

TERMS

• taphonomy
• taphonomic process
• biological process
• geological process
• postmortem
• gnawing
• carnivore
• exfoliation
• delamination
• longitudinal fracturing

• Carefully handle the bones laid out in the lab, being sure to keep the bones on bubble wrap with their provenience labels.

• Use gloves when handling real bone specimens.

• Hand lenses are available for low magnification examination of the skeletal material.  Use a strong light or sunlight.

• Record your responses to the questions on the answer sheet provided.  Pencil is suggested for recording your answers.

• Use reference books in the lab as needed.

• The lab is due on Thursday, March 20 at the start of class.  Late labs will not be accepted.

Taphonomy is the study of the fossilization process or the biological or geological processes that affect the condition, preservation or location of skeletal remains after an individual dies.  By definition, taphonomic processes are postmortem, meaning they affect the hard tissues after
death.  The effects of taphonomic processes on bones must be distinguished from skeletal attributes used in developing a biological profile.

BIOLOGICAL PROCESSES

Biological taphonomic processes are related to plant and animal activity and include rodent gnawing, carnivore gnawing, carnivore digestion, algal growth and root etching.

Rodent gnawing leaves paired, U-shaped parallel incisions on bones.  Rodents typically gnaw on the projections of bones (crests, tuberosities, condyles, processes, etc.), but the incisions also occur on low-relief surfaces of bones.  The fact that rodent gnawing marks are paired distinguishes them from carnivore gnawing, and the fact that rodent gnawing marks follow the contour of a bone distinguishes them from cut-marks or incised sharp-force traumas.

When rodents gnaw in opposite directions on a bone surface, they may create what appears to be a ridge or crest, as pictured in the upper left of the bone below.

Carnivore gnawing leaves single, U-shaped incisions or "channels" on bones that should not be confused with incised sharp-force traumas.  Carnivores also leave small round depressed fractures where their canine teeth puncture the cortical bone.  In addition, they gnaw the ends of bones and bone projections, exposing spongy bone; carnivores crush bone shafts in order to extract marrow, creating "spiral" fractures.  Note the punctures on the bone below.

Carnivore digestion results in pitting and dissolution of bone, which often exposes spongy bone.  A bone that is pitted by carnivore digestion is shown below.  This taphonomic alteration should not be confused with periosteal reactions.

Algal growth is evidenced by the accumulation of discontinuous green spots or continuous green areas on bone surfaces.  Algae grows on bones in moist, warm environments.  It should not be confused with copper staining from buttons or other evidence.  The arrow on the rib fragment below points to green algal growth.

The weak acids released by plants through their roots can leave root etching marks on bone surfaces.  The etching is typically into the cortical bone and forms a dendritic pattern. The etching actually creates subsurface linear patterns on the bone surfaces, though in the photo below of an etched sheep mandible it appears that the alteration is deposited on top of the bone surface.

http://hoopermuseum.earthsci.carleton.ca/taphonomy/BONMOD4.HTM

GEOLOGICAL PROCESSES

Geological taphonomic processes relate to erosional and weathering agents and include element/mineral deposition, water erosion, wet-dry or freeze-thaw cycling, sun exposure, and burning.

Elements and minerals may be deposited on bone surfaces as a result of precipitation of those materials from groundwater contact.  Common deposits include manganese, calcite or calcium carbonate, and iron or iron oxides.

Manganese deposition is characterized by discontinuous, amorphous patches of dull black material on bone surfaces.  It should not be confused by carbonization from burning, which is usually more glossy or reflective.

Calcite (calcium carbonate) deposition is characterized by discontinuous or continuous, amorphous patches of grayish, brittle, flaky material on bone surfaces.  It should not be confused with arthritic osteophyte development or dental calculus.

Iron and iron oxide deposition is characterized by discontinuous, amorphous, rust-colored or blackened areas or spots on bone surfaces.  It should not be confused with carbonization from burning.

Water erosion results in rounding and smoothing of bone edges and projections as well as cortical bone loss.

Repeated cycles of wet-dry or freeze-thaw conditions cause exfoliation or delamination, or the loss of cortical bone as well as the development of longitudinal fractures, which are breaks oriented parallel to bone shafts or surfaces.  The images below represent exfoliation and longitudinal fracturing.

Exposure to the sun causes bones to become bleached or whitened, such as the cow skull below.

http://www.istockphoto.com/file_thumbview_approve/187260/2/Where_s_the_Beef_.jpg

Natural fires can alter bones in the same ways as vehicular or residential fires.  Bones initially become fractured and blackened or carbonized (as shown below on animal bone).  As the burning progresses, bones become increasingly fragmented, fractured and whitened or calcined.

ASSIGNMENT

Using the above descriptions and comparative materials in the lab, examine each of the real bone specimens.  For each specimen, identify the skeletal element and side (for paired bones).  Identify the taphonomic agent(s) that affected the bones and, if asked, describe the effects (e.g., type of bone alteration, location of alteration, extent of alteration) of each agent.
 

REFERENCE

White, T. D. (1991) Human Osteology. Academic Press, San Diego.

Unless otherwise noted, all photographs are by Darlene Applegate.