Pogonophora are the previously unknown link in the evolutionary sequence connecting annelid worms to vertebrates via the hemichordates. Some items of evidence not mentioned in earlier posts on this topic include information compiled by Libbie Hyman and presented in her 1959 volume on The Invertebrates, volume V, 1959. The link status demonstrates the error of the widely assumed evolutionary separation of deuterostomes from advanced protostomes.
The confusion of the features of pogonophorans blending features of protostomes and deuterostomes was easily dismissed as caused by lack of adequate availability of well preserved specimens or some such estimation of the recently discovered group.
On page 59, she noted that, prior to the recognition of the pogonophorans as a phylum, one was described as "a vermiform animal with a crown of tentacles and was considered by its describer to be a polychaete annelid of the family Sabellidae".
Also, on page 59 she says "These outstanding studies by Ivanov have shown that the pogonophores are closely related to the hemichordates and belong among the Deuterostomia."
Webb's (1964) discovery of the segmented posterior of a pogonophoran helped endorse their annelid origin. Later, the contrary view was demonstrated by Gans and Northcutt (1983) affirming their connection to the deuterostomes despite the obvious annelid connection demonstrated by the generally unknown work of Webb.
So, Gould's (2002) contention that the evidence for elimination of the annelid theory should be dismissed means that the annelid theory should be reinstated as a leading explanation for the inversion of annelid features shown in the chordates. [ http://evolutioninsights.blogspot.com/2013/06/evolution-protostome-deuterstome-link.html and http://evolutioninsights.blogspot.com/2015/05/evolution-annelids-to-chordates-middle.html ] The second of the two posts just mentioned illustrates the larval similarity of pogonophoran larvae to solitary hemichordates. Hyman, on page 219, notes Caullery's 1944 observation that one pogonophoran's developing young "strikingly resemble young stages of the buds of Cephalodiscus", a colonial hemichordate.
I have tried to clarify the protostome-deuterostome connection demonstrated by the pogonophorans in numerous posts on this blog. By the time I realized the connection existed when provoked by the 1983 assertions of Gans and Northcutt, I had already published two revisions of Hegner's invertebrate zoology text, one in 1968 the other in 1981.
THE EMBRYOLOGICAL SHIFT FROM PROTOSTOME TO DEUTEROSTOME
The stumbling block for accepting the link is the drastic change in embryology that occurs. I might not have realized how it occurred if I had not revised Hegner's Invertebrate Zoology for its second edition (1968). In the process I had to deal with the proper classification of pogonophorans and realized the affinity that seemed impossible.
I tend to think there are answers for everything. I did not have the answer, but the needed facts were already in my background. As a result of the book preparation I had come to the conclusion that pogonophorans were extremely long-lived. [ http://evolutioninsights.blogspot.com/2015/05/abyssal-ocean-environment-and-extreme.html ] I did publish that conclusion in a 1978 abstract (Engemann, J. G., Indirect evidence shows deep-sea benthos may reach extreme ages as individuals. Am. Zoologist, 18:666).
My 1963 doctoral thesis at Michigan State University made me aware of the extreme affects environment can have on selection and evolution. I had not made much attempt to publish bits of the thesis because they were not greatly different from many other studies. But it made me aware of some necessary bits of information that helped explain the way embryological shift had been expedited.
The peculiar egg appendage of the isopod Asellus had been illustrated in Sar's work on the crustacea of Norway in the 1800's. K. H. Barnard had studied phreatoicid isopods in South Africa in the early 1900's and noted a bulge in embryos in a homologous position with the egg appendage that persisted briefly after the embryo hatched. My sections of phreatoicid eggs showed a thin-walled, yolk-filled homologue of the appendage existed before hatching as described in - http://evolutioninsights.blogspot.com/2014/06/egg-appendages-of-michigan-isopod.html .
The big thing to take from the above is that embryological features can evolve separately from the rate of adult features. Thus the old presumption that the earliest features in the evolution of the embryology of an organism correspond to the older ancestral stages in its evolution is not always true.
The thesis comparison of Tasmanian and Michigan isopods from comparable latitudes but quite different ecological circumstances in temporary pools showed drastically different rates of development and how r- and K- selection can work before the theory was described (MacArthur, R. H., and E. O. Wilson. 1967. The Theory of
THE MOLECULAR DATA
The exquisite work molecular biologists can do is remarkable. But it is limited and not easily applied to discovering ancestral relationships at the phylum level. Those limitations have been discussed in the post - http://evolutioninsights.blogspot.com/2013/05/science-screw-up-no-1.html .
My understanding of the slow rate of DNA changes in the pogonophora was confirmed by seeing how they showed up in phylogenetic studies among clusters of disparate groups due to little change in the over half-billion year old group almost in suspended animation in the abyssal sediments. Unfortunately, the generation time error affecting molecular clocks has not been considered in most work on molecular phylogeny.
What do you call a missing link, such as the pogonophorans, once they are discovered?
I never expected to have work on an obscure invertebrate lead to the above understanding. But, in retrospect, I realize that much credit goes to unknown biologists and scientists of the past as well as many known ones. Thanks to Dr. V. Hickman of the University of Tasmania for calling the phreatoicid isopod to my attention, Dr. T. W. Porter of Michigan State University and the rest of my doctoral committee for keeping me focused on the comparative thesis study, the U. S. Fulbright Agency for the award for study at the University of Tasmania, Western Michigan University for allowing me to teach a ridiculous range of classes whose subject matter provided some needed insights, and the Macmillan Publishing Company for contracting with me to revise their texts. A book would be needed to explain how students, family, friends and strangers contributed to my development under the guidance of the same Creator that made evolution a reality.
Joseph Engemann Kalamazoo, Michigan October 24, 2015