Species

Other Species Concepts 263

... named monophyletic groups which are identified by unique shared similarities

(apomorphies) ... which are at present not further subdivided. ... Identification of taxa

as LITUs are statements about the current state of knowledge (or lack thereof) without

implying that they have no internal nested structure; we simply do not know if a given

LITU consists of several monophyletic groups or not.^55

However, as the historical reviews above make abundantly clear, the notion of

a least inclusive taxonomic unit historically is a species in both logic and biology.

What Pleijel has done here is rediscover the past way of looking at things. However,

LITUs are ontological units, and not merely epistemic ones.

Species Concepts in Paleontology (Paleospecies)

The problem of applying any concept of species in paleontology has long been
understood, and has spurred many discussions.^56 The difficulty lies in the way the
data are presented. In neontology (the study of living organisms), the behaviors of
organisms, both sexual and ecological, can be observed in some detail and repeated
if the initial observations are inadequate. Under some circumstances, the organ-
isms can be experimentally mated. Molecular evidence, especially that of DNA, can
be harvested and assayed, meaning that where a traditional taxonomist might have
used at most around 40 characters, the molecular systematist has more like 40,000.^57
Also, polytypy can be investigated in extant species, enabling the investigator to
delineate the populations and subspecific variants and to tell whether, on the concept
used, these still are included in the specific group.
Not so the paleontologist. The information in that case is usually restricted to
several individual specimens (except when the bulk of the population leaves fossils,
as in the case of silaceous forams, whose shells are fossilized in sediments on the
seafloor, and which when they are found provide information about distribution,
variety in populations, and changes over time). The famous Tyrannosaurus rex, for
example, is known from around 50 specimens, none complete. Many hominid spe-
cies are known from a single individual. In cases where the ancestors of a lineage
lived in conditions unconducive to fossilization—the study of which is known as
taphonomy—will have entire series of species unknown to science. For example, few
ape fossils have been found for those species that lived in forest and jungle environ-
ments, where decomposition in the acidic soils, scavengers, and plants will dispose
of the carcass relatively quickly.
So, paleontologists rely almost exclusively on morphological data. This means that
there is pressure on them to lump stratigraphic specimens together (since fossil taxa
are often used as stratigraphic markers by geologists) if there is some subjectively

(^55) Pleijel and Rouse 2000, 629. See also Pleijel 1999, 2003.
(^56) Simpson 1943, Sylvester-Bradley 1956, Schopf 1972, Smith 1994.
(^57) A point made at the Melbourne Systematics Forum during 2002. I did not catch the name of the
person making the point, but it is important. In the end, unless we know the ways in which DNA
is expressed developmentally in all the species being analyzed, DNA is just a richer source of
“morphological” data. However, few of these nucleotides are likely, in practice, to be informative
(D. Williams, pers. comm.).