Geoscience Reference
In-Depth Information
faster bone deposition in the set from Niederkirchen
than in that from Odernheim.
period
of
torpor
(hibernation
or
aestivation)
annually.
Erdesbach (L-O-7-horizon). One specimen of
A. pedestris from Erdesbach presents a simple-LAG
pattern (Fig. 3d). However, all the studied speci-
mens of A. caducus from Erdesbach have limb-bone
diaphyseal cortices that sometimes express simple
LAGs and, in other circumstances, double LAGs.
In the latter case, a pattern showing distinct pairs
of LAGs (Figs 2b & 3b, d) can be observed,
suggesting two arrests of growth per year. Such a
double-LAG pattern has already been observed in
extant populations of T. marmoratus from high
elevations that undergo two periods of drastic
environmental conditions every year, forcing them
to annually hibernate and aestivate (Caetano et al.
1985; Caetano & Castanet 1993). This suggests
that the specimens from Erdesbach presenting a
double-LAG pattern needed to stop their growth
twice per year (to aestivate and hibernate) because
of peculiar and perhaps drastic environmental con-
ditions (Figs 2b & 3b, d).
Skeletochronological analysis
Growth marks are well preserved at the diaphyseal
level (Fig. 3), allowing a skeletochronological
analysis. Allochronies (Castanet et al. 1996a)
between limb bones in Apateon do not permit a ske-
letochronological analysis of every bone. Thus, only
stylopod long-bones (humerus and femur) of
Apateon species have been indifferently used as
bones of reference to account for the total number
of growth marks. Lines of arrested growth
(LAGs), usually bordering each growth mark, are
structures expressing a stop during the osteogenesis
followed by a sudden resumption of growth. They
are expressed among living poikilotherms during
annual aestivations and/or hibernations (Castanet
1985; Castanet et al. 1993). LAG patterns in long
bones of Apateon are similar to those of living tetra-
pods, indicating that one LAG in Apateon may also
have been the consequence of a quiescent osteogen-
esis occurring during a lethargic period. Usually,
extant tetrapods only enter into a lethargic period
once a year (Castanet et al. 1996b; Bruce & Castanet
2006). They therefore express annually a so-called
'simple-LAG pattern'. However, it was shown that
some extant newt populations of Triturus marmora-
tus hibernate and aestivate each year (e.g. Caetano
& Castanet 1993). In this case, such cyclical life
habits are reflected by a peculiar spatial pattern of
growth marks in the limb bones called the 'double-
LAG pattern'; this indicates two phases of growth
and lethargy per year (Caetano et al. 1985).
Niederkirchen (L-O-6-horizon). All the specimens
of A. pedestris from Niederkirchen show a double-
LAG pattern (Figs 2c & 3c, d), which again suggests
that
they
hibernated
and
aestivated
throughout
their development.
Determination of the individual age
The age of these fossil individuals can be assessed
by skeletochronology in the same way as it is used
to estimate the somatic age of living tetrapods.
Indeed, the LAG patterns observed in long bones
of Apateon's individuals are identical to that of
small extant tetrapods. This allows us to consider
that one growth mark, that is, one LAG and the
immediately neighbouring bone layer record, is
the time during which a seasonal and biological
cycle was achieved; this is usually equivalent to
one calendar year (Castanet et al. 1993). However,
there are peculiar ecological situations in which
one biological yearly cycle is split by two close
arrests of osteogenesis, that is, expressed by two
close LAGs in extant tetrapods (corresponding to
aestivation and hibernation; e.g. Caetano et al.
Identification of LAG patterns in individuals
from different localities
Odernheim and Rehborn (L-O-8-horizon). All the
sampled specimens from Odernheim (n ¼ 6) and
Rehborn (n ¼ 1) belong to A. pedestris (Table 1).
They all show a simple-LAG pattern (in humerus
and femur; Figs 2a & 3a, d), reflecting only one
quiescent phase of osteogenesis per year. Conse-
quently, these specimens underwent only one
reduction of growth-rate per year, expressing a
Fig. 3. (Continued ) Annual biological cycles indicated by LAGs (e.g. (a) SMNS 54988, (b) GPIM-N 1572 and
(c) SMNS 55018) are highlighted by the small black and white arrows. When the cortices are incomplete, the position
of LAGs of annual cycles are reconstructed by dotted lines, thereby allowing a retrocalculation to estimate the
somatic age. (d) Comparison of the different LAG patterns observed in limb bones of few samples of Apateon taxa
(A. pedestris and A. caducus) from the Permian of the Saar-Nahe Basin, Germany. Individuals from Niederkirchen
(humerus of SMNS 55018) always present a double-LAG pattern; from Erdesbach (humeri of GPIM-N 1389 and
GPIM-N 1572) both a double- and simple-LAG patterns; from Odernheim (femur of SMNS 54980) and Rehborn
(femur of MB.Am.1217) always a simple LAG pattern (scale bars of thin sections: 0.1 mm).
