Geoscience Reference
In-Depth Information
found on islands located sufficiently far from the
nearest continent to have few insects and arachnids
(such as Guadeloupe; personal observation),
although some occur on the continent such as
Potamon in the Alborz range (Iran; P. Janvier,
pers. comm., 2009). Yet, several primitively
marine crustaceans have perfectly functional
walking appendages that can be used to walk on
land with little or no modification. In this respect,
it should be less difficult for arthropods to adapt to
terrestrial locomotion than for teleosts, whose
paired fins are poorly suited for this task. Despite
all this, very few crustaceans have invaded inland
habitats,
mudskippers, most of which are less than 15 cm in
length (Graham & Lee 2004, p. 727). Thus, the
reliance of extant tetrapods on lung ventilation is
not a strong argument against their origin from
coastal areas because the lung is probably an
osteichthyan synapomorphy (Sullivan et al. 1998).
It is not established that Devonian stegocephalians
had larger or more complex lungs than their finned
sarcopterygian relatives. Lung complexification in
tetrapods may have occurred shortly before the
origin of the crown-group, whose composition is
controversial (Laurin 1998a, b; Ruta et al. 2003;
Vallin & Laurin 2004; Ruta & Coates 2007) but
which probably appeared only in the Early Carbon-
iferous. Under some topologies, a terrestrial life-
style may have been acquired in stem-tetrapods
well before the origin of the crown because sey-
mouriamorphs and several temnospondyls, which
may be stem-tetrapods, appear to have had terres-
trial adults (Sumida et al. 1998; Sullivan & Reisz
1999; Laurin 2000; Laurin et al. 2004).
This brief discussion suggests a marginal-marine
origin of terrestrial vertebrates, and reveals weak-
nesses in arguments that were presented to refute
this hypothesis. However, the large amount of
uncertainty in the data plainly shows that much
additional work is required to reach a well-
corroborated resolution. This will probably not be
easy because similar controversies affect the
habitat of other Palaeozoic taxa such as ostracodes;
the oldest (Devonian) occurrence of that taxon in
presumed freshwater is partly supported by associ-
ation with 'freshwater fishes' (Friedman & Lundin
2001, p. 73)!
presumably
because
numerous
insects
and arachnids already occupy these habitats.
This suggests that the failure of mudskippers
(Periophthalmidae and close relatives) and other
amphibious teleosts to become more fully terrestrial
may reflect competitive exclusion, rather than
intrinsic limitations of their bauplan or incompatible
evolutionary pressures exerted by the intertidal
environment. Another possibility is that these tele-
osts never acquired metabolic adaptations as good
as those found in tetrapods to deal with nitrogen
excretion outside the water. This possibility is
raised by recent works which shows that most
amphibious teleosts are ammonotelic (they
produce ammonia, which is toxic and difficult
to excrete in air) rather than ureotelic (Ip et al.
2004, p. 774). One of the few exceptions is Peri-
ophthalmus sobrinus, which excretes about as
much urea as ammonia and can shift towards ureo-
telism when out of the water (Gordon et al. 1969).
The infrequent occurrence of ureotelism in amphi-
bious actinopterygians appears to be linked partly
to its metabolic cost, which may be prohibitive in
most teleosts species which developed alternative
strategies for dealing with nitrogenous waste when
out of the water (Ip et al. 2004).
Finally, Graham & Lee's (2004) analysis seems
to rest on the hypothesis of 'tetrapod land life selec-
tion being driven by alternating (likely seasonal)
periods of rain and drought.' That scenario was
popular through much of the 20th century but has
been discarded because, among other reasons, the
redbeds on which that hypothesis rests are now
known not to require seasonal aridity to form (Czys-
cinski et al. 1978; Laurin et al. 2007). This therefore
deprives Graham & Lee's (2004) hypothesis from
geological support.
Other arguments against a marine origin of ter-
restrial vertebrates proposed by Graham & Lee
(2004) are similarly of limited value. Their argu-
ment (p. 727) that the waves exert an evolutionary
pressure to increase body density is interesting,
but it would not apply to most mangrove and lagoo-
nal habitats. It might also apply less to large taxa
(about 1 m body length) than to the much smaller
We thank P. Janvier and G. Cl
ยด
ment for numerous sugges-
tions which substantially improved the text. This research
was funded by the CNRS and the French Ministry of
Research (grants to UMRs 7179 and 7207). Funds for
work in the Puertollano basin were provided by DGES pro-
jects
PB95-0398
and
PB98-0813
(MEC,
Spain)
and
BET2002-1430
(Spanish
Science
and
Technology
Ministry).
References
Ahlberg, P. E. 1998. Postcranial stem tetrapod remains
from the Devonian of Scat Craig, Morayshire, Scot-
land. Zoological Journal of the Linnean Society, 122,
99 - 141.
Ahlberg,P.E.&Milner, A. R. 1994. The origin and
early
diversification
of
tetrapods.
Nature, 368,
507 - 514.
Anderson, J. S. 2001. The phylogenetic trunk: maximal
inclusion of taxa with missing data in an analysis of
the Lepospondyli (Vertebrata, Tetrapoda). Systematic
Biology, 50, 170 - 193.
Anderson, J. S. 2007. Incorporating ontogeny into the
matrix: a phylogenetic evaluation of developmental
evidence
for
the
origin
of
modern
amphibians.
