As crustacean metazoans, ostracods have specialised tissues, and organ systems.
They are typically larger than recent foraminiferans, mostly in the 0.3–5 mm range,
although some marine species exceed 30 mm in length. The most distinctive feature
of ostracods is their calcified carapace comprising paired, dorsally articulated valves
(Meisch 2000). Both groups, Ostracoda and Foraminifera, have a marine origin,
have a test or shell saturated with calcium carbonate, and inhabit aquatic environments. Both have undergone spectacular adaptive radiation and colonised wide
variety of marine habitats. In the Early Carboniferous ostracods and foraminiferans
thrived. It is likely that during peak marine transgression both groups had opportunities to colonise freshwater, inland habitats. Ostracods succeeded and foraminiferans seemed to be less fortunate, but why?
2 Evolution from the Palaeozoic to the Recent
Traditionally, the evolution of Foraminifera is viewed as a gradual process of
change in structure and composition of the test, starting from naked, unilocular
forms, via organic-walled and agglutinated forms which later became multilocular,
and ending with the highly complex calcareous forms (Tappan and Loeblich 1988).
Recent molecular studies (Pawlowski and Holzmann 2002; Pawlowski et al. 2003),
however, revealed that there is no evidence for a progressive increase in foraminiferal test complexity. In a single highly supported clade, for example, a distinctive
radiation included a wide variety of test morphotypes containing both agglutinated
(Textulariida) and calcareous (Rotaliida) species. In a recent study Pawlowski et al.
(2013) showed that transition from organic to agglutinated walls occurred several
times, and the change in the nature of test wall was dependent on environmental
conditions. According to these authors a calcareous wall appeared at least five times
independently, and each time a different type of calcareous test was developed. It
appears that some species may have lost their calcified test secondarily, for instance,
as an adaptation to the lower salinity regime.
The first fossils of Foraminifera are agglutinated Textulariina and appeared
during the Cambrian (McIlroy et al. 2001; Boudagher-Fadel 2008). They remained
as a dominant group until the Silurian when the larger, calcareous and more complex
Fusulinina appeared, becoming abundant in the late Palaeozoic. In the Silurian, apart
from textulariines and fusulinines, a new group of Foraminifera with the test wall
consisting of calcite crystals evolved—the Lagenida. Less significant ecologically in
the Palaeozoic were the Miliolida (having porcelaneous test) and early Involutinina
(with an aragonitic wall). More advanced calcareous tests, formed by biomineralisation of an inner tectinous lining, may have appeared by the Silurian, but did not
become widespread until the late Devonian (McIlroy et al. 2001).
The Palaeozoic seems to be a period of fusulinines. They underwent rapid
evolutionary radiation from tiny, simple organisms to large, complex and highly
specialised forms in diverse lineages (eight known families) (Boudagher-Fadel
2008). Fusilinines became ubiquitous and spread to most warm, shallow waters
The Adaptations of the Foraminifera and Ostracoda …
93
They are typically larger than recent foraminiferans, mostly in the 0.3–5 mm range,
although some marine species exceed 30 mm in length. The most distinctive feature
of ostracods is their calcified carapace comprising paired, dorsally articulated valves
(Meisch 2000). Both groups, Ostracoda and Foraminifera, have a marine origin,
have a test or shell saturated with calcium carbonate, and inhabit aquatic environments. Both have undergone spectacular adaptive radiation and colonised wide
variety of marine habitats. In the Early Carboniferous ostracods and foraminiferans
thrived. It is likely that during peak marine transgression both groups had opportunities to colonise freshwater, inland habitats. Ostracods succeeded and foraminiferans seemed to be less fortunate, but why?
2 Evolution from the Palaeozoic to the Recent
Traditionally, the evolution of Foraminifera is viewed as a gradual process of
change in structure and composition of the test, starting from naked, unilocular
forms, via organic-walled and agglutinated forms which later became multilocular,
and ending with the highly complex calcareous forms (Tappan and Loeblich 1988).
Recent molecular studies (Pawlowski and Holzmann 2002; Pawlowski et al. 2003),
however, revealed that there is no evidence for a progressive increase in foraminiferal test complexity. In a single highly supported clade, for example, a distinctive
radiation included a wide variety of test morphotypes containing both agglutinated
(Textulariida) and calcareous (Rotaliida) species. In a recent study Pawlowski et al.
(2013) showed that transition from organic to agglutinated walls occurred several
times, and the change in the nature of test wall was dependent on environmental
conditions. According to these authors a calcareous wall appeared at least five times
independently, and each time a different type of calcareous test was developed. It
appears that some species may have lost their calcified test secondarily, for instance,
as an adaptation to the lower salinity regime.
The first fossils of Foraminifera are agglutinated Textulariina and appeared
during the Cambrian (McIlroy et al. 2001; Boudagher-Fadel 2008). They remained
as a dominant group until the Silurian when the larger, calcareous and more complex
Fusulinina appeared, becoming abundant in the late Palaeozoic. In the Silurian, apart
from textulariines and fusulinines, a new group of Foraminifera with the test wall
consisting of calcite crystals evolved—the Lagenida. Less significant ecologically in
the Palaeozoic were the Miliolida (having porcelaneous test) and early Involutinina
(with an aragonitic wall). More advanced calcareous tests, formed by biomineralisation of an inner tectinous lining, may have appeared by the Silurian, but did not
become widespread until the late Devonian (McIlroy et al. 2001).
The Palaeozoic seems to be a period of fusulinines. They underwent rapid
evolutionary radiation from tiny, simple organisms to large, complex and highly
specialised forms in diverse lineages (eight known families) (Boudagher-Fadel
2008). Fusilinines became ubiquitous and spread to most warm, shallow waters
The Adaptations of the Foraminifera and Ostracoda …
93