Spring 2004
Lecture 14: March 30
All Lophophorates have a lophophore: a set of tentacles shaped in a ring, horsehoe or coil. covered with cilia that generate a current of water that flows toward the mouth. The tentacles are hollow, with the space a part of the body cavity (coelom). The mouth is inside the lophophore, the anus just outside the lophophore in Phoronida, Ectoprocta and inarticulate brachiopoda, whereas articulate brachiopoda have no anus and in Entorprocta it is inside the lophophore.
3.
Entoprocta
(Kamptozoa, goblet worms)
Cladogram of Brachiopoda (compare with that in text book, p. 233)
Traditionally, the brachiopods have been split into two major groups, the Inarticulata and the Articulata. The Inarticulata got their name from the fact that they do not have a hinge. The Lingulata and Obolellida have been placed in the Inarticulata, but there is disagreement about their placement, and that of several other groups (see textbook p. 233). Recently it has been argued that hinge articulation is not a good character on which to make brachiopod groups (note that the various Inarticulate groups are not sister groups in the cladogram). Various authors proposed that brachiopods are polyphyletic, and that different brachiopod groups evolved separately from phoronid-like ancestors.
Articulata: hinge, simple muscle system
Dominant in Paleozoic, from Ordovicium through Permian. Major extinction at end Permian, although a few groups lingered on through the end of the Triassic.
Bivalves have 'won'; they are everywhere in the oceans. In Paleozoic: Brachiopoda more common, abundant and ubiquitous in the oceans, particularly in outer continental shelf and off the slope. Bivalves: in Ordovician only in shallowest waters, close to the beach; during the Paleozoic expanding somewhat to middle shelf (see text book p. 142). After Permian extinction: bivalves took over most of the ocean habitats, brachiopods hanging on in non-favorable habitats (e.g., very little food, low oxygen). Why was this?
Example of major question: what is the role of competition in macro-evolution ?
We can only determine whether one group out-competed another if we have a good fossil record of groups that live in the same habitat, and whose modes of life overlap, otherwise there would not be any real competition.
Was there indeed competition? or were these two groups 'ships that pass in the night' . S. J. Gould and C. B. Calloway (1980, Clams and brachiopods: ships that pass in the night. Paleobiology, 6, 383-396) argued that the fossil record indicates the latter. If the two groups had been in constant competition (and if the global environment had only a fixed number of places for clams plus brachiopods, the carrying capacity for that type of organism), the diversity of bivalves should have increased while that of brachiopods decreased. This may not have been the case (figure with data from Gould & Calloway, 1980, plotted are number of brachiopod versus number of bivalve genera).
IF there had been competition, one would expect (simplistically) a negative correlation between the number of brachiopods and the number of bivalve genera present. Note that there seems to be a positive correlation between number of genera of both groups (more Molluscan genera occur together with more Brachiopoda genera), and just a very different relation for Paleozoic and post Paleozoic times. This (not very convincing) positive correlation most likely results from the fact that the numbers of genera for both groups are given per geological time period (of varying length) rather than per unit of time (e.g., million years). Longer periods thus tend to have more genera of both groups. We just do not have data in precise time units for most intervals of time. If we look at these same data (Gould & Calloway, 1980) plotted versus numerical age, we get the following:
Gould & Calloway concluded that the Molluscan Bivalves radiated after the extinction of many of the Brachiopoda at the end of the Permian into an 'empty ecosystem', and that there was thus no direct competition: one group was affected much more severely than the other in the mass extinction, which was indiscriminate.
But one could also look at these same data, and conclude that the bivalves would have out-competed the brachiopods even if there had not been an end Permian extinction: after all, the Molusca were slowly increasing in diversity during the Paleozoic anyway, and they might just have been helped along a bit by the extinction. Jack Sepkoski argued that a somewhat more mathematically sophisticated treatment of the data (plus new data) strongly suggests that there was, indeed, a major component of competition, and that the mass extinction only interrupted that process to some extent, but did not really change the final outcome in any way (J. Sepkoski, 1996, Competition in Macroevolution: the double wedge revisited, In: D. Jablonski et al., eds., Evolutionary Paleobiology, University of Chicago Press, p. 211-255). Figure from Sepkoski 1996, modeling the Brachiopod (clade 1) and Bivalve (clade 2) abundances of genera. Note the downward-trend in Brachiopod diversity, upward trend in bivalve diversity even before the mass extinction at 250 Myr (model time).
So: why did the Bivalves (clams) gain the upper hand, rather than the Brachiopoda recover and regain dominance? There is no agreement.
These three possibilities are not the only ones, and they are not mutually exclusive. It thus remains an open question whether Bivalves would ever have out-competed the Brachiopods if there never had been an end-Permian extinction. We do not know if the bivalves could ever have succeeded in out-competing the well-established, widely distributed Brachiopoda in the deeper parts of the oceans if the latter had not been decimated by the extinction.