Biology Reference
In-Depth Information
landmarks
are located at points of sharpest curvature along tissue boundaries. Jugale, the
deepest incurvature along the posterior edge of the zygomatic between the frontal and
temporal processes (
Howells, 1973
:175), is an example of a Type II landmark.
Type III land-
marks
are relational points whose placement is dependent on the location of another land-
mark. These are often endpoints of measurements like maximum cranial breadth (euryon)
and maximum cranial length (opisthocranion). Problems with Type III landmarks include
a lack of biological meaning independent of the other landmark/endpoint and the high
degree of variability in the landmark location from individual to individual.
Type I and II landmarks are fairly common on the cranium and provide relatively complete
coverage of the craniofacial form. The same cannot be said of the postcranial elements where
most landmarks tend to be Types II and III. The emphasis of the cranium in the application of
geometric morphometrics is not surprising, given the ease of identifying biologically mean-
ingful landmarks on the cranium and that craniofacial morphology has already been the focus
of traditional morphometric research in skeletal biology for several decades.
Figure 12.1
depicts common cranial landmarks employed in geometric morphometric
research. For definitions of these landmarks, refer to
Howells (1973), Moore-Jansen et al.
(1994)
, and
Buikstra and Ubelaker (1994)
. It is strongly suggested that you carefully read
the landmark definitions found in the original sources listed above before trying to locate
them on a cranium, as you cannot always fully appreciate the intended location of a landmark
from two-dimensional images such as
Figure 12.1
. The definitions are detailed and standard-
ized to decrease the likelihood of observer error in landmark placement. It is critical that you
be well-versed in the anatomical features that define a particular landmark and understand
how to handle exceptions from expected morphology.
RECORDING LANDMARKS
Landmark coordinates can be recorded from a photograph or radiograph of a specimen
with a two-dimensional digitizing tablet or from a three-dimensional digitizer, such as
a Microscribe (
http://www.3d-microscribe.com/
)
or Polhemus (
http://polhemus.com/
).
Three-dimensional digitizers can also be used to observe coordinates directly on a dry
bone specimen (see
Figure 12.2
). While these types of digitizers will transmit coordinates
to programs such as Excel, Notepad, or AutoCad, there are also specialized software
programs to facilitate data collection. Landmark data can also be acquired from laser surface
scans and volumetric (computed tomography (CT) or magnetic resonance imaging (MRI))
scans. For more on these technologies, see Moore (Chapter 14), this volume.
A popular Windows-based program for acquiring landmark coordinates and craniomet-
rics from the cranium and mandible is 3Skull written by Stephen Ousley. An early version
of this program is described in
Ousley and McKeown (2001)
, and it is designed to work
with a three-dimensional digitizer such as the Microscribe or Polhemus. The program
provides a straightforward interface page for recording catalog and/or identification infor-
mation about the specimen, and then prompts the observer for each landmark to be digitized.
The basic list of landmarks provided in
Ousley and McKeown (2001)
includes all the land-
marks associated with the suite of
Howells' (1973)
craniometrics plus numerous other land-
marks to provide comprehensive coverage of important anatomical features of the skull.
The program is also designed to record continuous coordinates along a series of arcs
(frontal, parietal, occipital, and others). The arcs are used to calculate the location of some
