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

-97­
PRDIGED SIG EMS ElF GIV FRS FAM ITOU PRD IGED SIG EMS ElF GIV FRS FAM ITOU
SIL DEV CARB SIL DEV CARB
FIG. 6. - A comparison between the generic-level diversity patterns associated with MILES (1969) hypothesis of replacement
and the patterns among monophyletic placoderm groups. Note differences in timing of originations and the potential for
interactions between groups. A, arthrodiran diversity after MILES (1969): actinolepid level (squares), phlyctaeniid level (triangles),
coccosleomorph level (diamonds), and pachyosteomorph level (circles). B, monophyletic groups: AClinolepidoidei
(squares), Phlyctaenii (triangles), coccosteomorph arthrodires (diamonds), and pachyosteomorph anhrodires (circles).
Comparaison entre les modeles de la divers;"! des genres de Placodermes selon I'hypothese de MILES (1969) et les modeles
de groupes monophyletiques de Placodermes. Remarquez les differences entre Ie temps de l'origine des groupes et leurs
relations muwelles. A, diversile des arthrodires selon MILES (1969): actinoltipides (carris) .. phlyclaelliides (triangles) .. coccosteomorphes
(losanges) el pachyostiomorphes (cere/es). B, groupes monophyltiliques: Actinolepidoidei (carres), Phlyclaenii
(triangles), coccosleomorphes (losanges) et pachyosleomolphes (cere/es).
and Homosteidae ("primitive" brachythoracid, LELIEVRE, 1988) to be pachyosteomorph arthrodires.
Aspinothoracid arthrodires are considered to be monophyletic (MILES & DENNIS, 1979;
CARR, 1991; but contrast DENISON, 1984) and here include: Brachydeiridae, Bungartiidae,
Leiosteidae, Leptosteidae, Mylostomatidae, Selenosteidae, Titanichthyidae, Trematosteidae,
Gorgonichthys clarki, Heintzichthys gouldii, Holdenius holdeni (pers. observ.), and Dinichthys
herzeri (CARR & HLAVIN, MS).
A generalized sequence of temporal replacement can be seen among the four arthrodiran
clades as noted by MILES (1969) and GARDINER (1990); however, a competitive causal relationship
is not certain since these patterns represent global data. The pairwise patterns in each case
of putative competitive displacement do not demonstrate a clear pattern of ecological replacement
("double-wedge pattern," BENTON, 1987). Additionally, a causal relationship between MILES'
(1969) levels should be evaluated in the light of other placoderm and gnathostome taxa considering
alternative biotic or abiotic interactions. Actinolepid and phlyctaeniid patterns are not
consistent with competitive displacement (Fig. 6) with both groups sharing a similar history of
first appearance and peak diversity (Gedinnian and Siegenian respectively). Substage-level analysis
demonstrates a possible delay in the timing of phlyctaeniid peak diversity (Upper Siegenian
versus Lower Siegenian for the actinolepids). Also, it should be remembered that phlyctaeniids
may represent a paraphyletic assemblage needing further evaluation. Coincident with the decline
of phlyctaeniids and increase among coccosteomorphs was an increase among antiarchs, ptyctodonts,
pachyosteomorphs, and osteichthyans (Figs. 5, lOA). When using monophyletic groups