Geoscience Reference
In-Depth Information
15% PA L
30% PA L
250
200
150
100
50
0
0
30
60
90
120
Oxygen concentration (μM)
Figure 11.5. Relationship between fish size and their survival under various levels of oxygen.
Plotted is the LC50, and this represents the level where 50% of the population died during a
specified period of time. Averages and standard deviations are given when multiple examples
of the same species were studied. Bars are also shown giving the oxygen concentrations
corresponding to 15% and 30% of present atmospheric levels. Note that only small fish
survive at low oxygen levels under 15% PAL. From a data compilation kindly made available
by Emma Hammarlund.
better known as sea scorpions, are ancient arthropods that first appear
during the Ordovician Period. Some members of this clade grew to
sizes exceeding 3 meters near the Silurian-Devonian boundary, some
420 million years ago. Thus, one might argue, as Tais and Emma did,
that an increase in atmospheric oxygen levels, accompanying the rise of
land plants, led the way to fish (and eurypterid) gigantism in the oceans.
There were, however, some big swimming organisms in the oceans be-
fore the rise of land plants. For example, there is the Middle Cambrian,
meter- long
Anomalocaris
, as captured in the Burgess Shale of western
Canada and in the Chengjiang deposits of China. Also, the euryptid
genus
Megalogratidae
reached sizes of 1 meter in the late Ordovician Pe-
riod. There were also examples of several meter-long squid-like nauti-
loids (in the order
Endocerida
) of the Ordovician Period some 450 mil-
lion years ago. The presence of these early giants doesn't mean that Tais
