Geoscience Reference
In-Depth Information
The earliest radiation of tetrapods is seen as a
consequence of the increase in capacity of air
breathing of their closely related fishes during a
period of low oxygen rate (Clack 2007). After that
episode, Frasnian - Famennian tetrapods radiated
in a period of increasing oxygen level (Ward et al.
2006; Clack 2007) which might have some relation
to
fossil data from discontinuous surfaces of preserved
sediments that have been sampled in an incomplete
geographical frame (N. America, Europe, Australia
and China in the case of earliest tetrapods). As
pointed out by Young (2006), alternative hypoth-
eses to the consensus scenario may be envisaged.
It might be that the out-of-Euramerica scenario
(Clack 2002) is simply the scenario for western
palaeontologists.
When reviewing the biostratigraphical ages
of Late Devonian tetrapod-bearing localities, it
appears that this has impacts on interpreting
the Frasnian - Famennian (FF) biotic crisis and
Devonian - Carboniferous (DC) event. Re-dating
the Sinostega locality of N. China as Frasnian
implies that there is no post-Frasnian ostracoderms
(the galeaspid which comes from the same locality
as Sinostega being also Frasnian, not Famennian)
and that the FF crisis has been real for ostracoderms.
Re-dating the East Greenland Middle to Late Devo-
nian Old Red Sandstone sequence (Marshall et al.
1999; Blom et al. 2007) places the DCB in the
Obrutschew Bjerg Formation. This implies that
Groenlandaspis and Holoptychius still exist in the
very Early Carboniferous, which is astonishing
because placoderms and porolepiform sarcoptery-
gians are usually thought to have disappeared
before or at the DCB. This would mean that the
DC event was not a total extinction for placoderms
and porolepiforms, except if the DC boundary is
higher in the East Greenland sequence.
the
increasing
rate
of
diversification
of
the
vegetation (Streel 2007a, 2009).
The earliest tetrapods are contemporaneous with
a late Frasnian warm climate phase on West Gond-
wana and Euramerica while their second phase of
diversification in the late and latest Famennian
occurs in a period of rapid climatic changes
with fast (c. 100 - 200 ka long) sea-level changes
(Fig. 2; Streel 2007a, b). The latter may have
caused quick changes in the development of near-
shore marine platforms and had impacts on the
coastal plains and alluvial deposits with niches
favourable to early tetrapods. However, such co-
occurrences of events do not imply direct causal
relationships between them. The global situation
was certainly more complex and implied trophic
relationships between the tetrapods and their
locally associated fishes in very different environ-
ments from true freshwater to true coastal marine
(e.g. Lebedev 2004). Between the two phases of bio-
diversification, the lower-middle Famennian veg-
etation crisis phase (Fig. 2) was controlled by a
high climatic gradient between cool polar areas
and a very warm, intertropical area (Streel 1992).
This may have been unfavourable for the diversifi-
cation of tetrapods, which are rare at that time.
We thank P. E. Ahlberg (Uppsala University, Sweden)
who provided us with the latest phylogenetic analysis of
early tetrapods which was in press at the time of writing
this paper (Ahlberg et al. 2008) and A. A. Warren (La
Trobe University, Victoria, Australia) who sent a reprint
of Gouramanis et al.'s (2003) paper. P. Janvier (CNRS,
Paris) helped with information concerning the genus
Groenlandaspis. This paper is a contribution to IGCP
Project 491 (Middle Palaeozoic Vertebrate Biogeography,
Palaeogeography and Climate) and to the ECLIPSE II Pro-
gramme of CNRS 'The terrestrialization process: model-
ling complex interactions at the biosphere-geosphere
interface'. We also thank both referees J. A. Clack (Cam-
bridge University, UK) and E. B. Daeschler (Academy of
Natural Sciences of Philadelphia, Pennsylvania, USA).
Conclusions
The primary aim of this paper is to update the bios-
tratigraphical scale in which most of the Late Devo-
nian tetrapod-bearing localities worldwide can be
placed (Fig. 1). Uncertainties still remain for
several basal taxa such as Elginerpeton, Metaxyg-
nathus and Sinostega (if the latter is basal). As
several taxa are very incompletely known (either
solely as lower jaws e.g. Metaxygnathus, Sinostega
and the ichthyostegid from Strud, or as trackways)
they cannot yet be integrated in a general phyloge-
netic analysis (Fig. 2). Despite such limitations, a
consensus scenario arises where tetrapods seem to
have originated on Euramerica in the Middle to
Late Devonian transitional period. This was a time
of warm climate and increasing atmospheric
oxygen level, during the building of a pre-Pangaean
configuration of landmasses (Young 2006; Blieck
et al. 2007). As is now well known for the fossil
record in general (e.g. Sheehan 1977), such scen-
arios are the result of the interpretation of sparse
Note added in proof
While this paper was in the process of editing,
Niedzwiedzki et al. (2010) have published a series
of early Eifelian (Middle Devonian) tracks and
trackways from the northern Lysogory region of
the Holy Cross Maintains, Poland. This discovery
confirms the recent paradigm that the earliest tetra-
pods have to be found before the Late Devonian, say
at least in the Middle Devonian. This discovery as
