Carr, R. K., 1995a. - Biological Sciences

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stable levels until the Frasnian. Peak diversity was achieved in the Upper Frasnian followed by
a dramatic drop at the Frasnian-Famennian boundary (57-62%, table I). Figure 6 presents generic
diversities for Actinolepidoidei, Phlyctaenii, coccosteomorph, and pachyosteomorph arthrodires
based on both MILES' (1969) and current classifications (see discussion below).
PATTERNS OF DIVERSITY FOR OTHER GNATHOSTOMES
Chondrichthyans are first recorded from the Lower Silurian (NOVITSK;\YA & KARATAYUTE­
TALIMAA, 1986; KARATAYUTE-TALIMAA, 1992) with a significant increase in diversity at the
Devonian-Mississippian boundary (Fig. 7A). Approximately 50% of this increase is accounted
for by isolated teeth, denticles (dermal and mucus membrane), and other ichthyodorulites. Among
chondrichthyans, the Subterbranchialia (ZANGERL, 1981) and elasmobranchs have similar patterns
with elasmobranchs possessing greater numbers.
Acanthodians (Fig. 7B), with bimodal peak generic diversities in the Siegenian and Eifelian,
demonstrated a relatively gradual decline throughout the Devonian and Carboniferous until their
extinction in the Permian. A moderate increase in the rate of decline is noted at the Frasnian­
Famennian boundary. Among the three recognized orders of acanthodians (DENISON, 1979), the
climatiids and ischnacanthids appear to account for the earlier peak in diversity while climatiids
and indeterminate specimens account for the latter acanthodian peak. Low diversity among these
orders hinders further analysis.
Among sarcopterygians (taxonomy after CARROLL, 1988), Onychodontiformes, Holoptychoidea,
and Dipnoi first appear in the Lower Devonian (CARROLL, 1988; Fig. 8). Osteolepidoidea
first appear in the Middle Devonian (CARROLL, 1988) and are followed in the Frasnian by
coelacanthiformes and Tetrapoda. Sarcopterygians, recorded here at an epoch level of resolution,
demonstrated an increase in generic diversity from the Givetian to Frasnian. Diversity changes
during the Frasnian-Famennian extinction episode cannot be evaluated due to the lack of resolution.
In contrast, actinopterygians (Fig. 8B) showed little or no increase from first appearance
in the Middle Devonian (CARROLL, 1988) until the Tournaisian. During the final decline of placoderms
we see a similar decline in rhipidistians and dipnoans, although, not to complete extinction
with the exception of onychodonts. Coelacanth diversity remained relatively stable during
the Late Devonian and Early Mississippian, but again, this may reflect a lack of resolution.
DIVERSITY PATTERNS
DISCUSSION
Although the fossil record is a filtered view of "true" diversity (RAUP, 1979, notes biases
due to taxonomic level, geographic distribution, taphonomy, sampling, and rock availability),
this record represents the only source of information for extinct taxa like placoderms. In considering
evolution and extinction, it is important to recognize that diversity patterns reflect outcomes
of both physical and biological interactions. Additionally, in evaluating global diversity
it is important to consider patterns of phylogeny and geographic distribution. With the above