In 1997, when I retired from the Zoology faculty at the University of Texas at Austin, I turned my attention from experimental embryology to efforts to examine the origin of the vertebrates. Deducing evolutionary pathways requires information from wherever one can get it. Paleontology, comparative anatomy of extant organisms, embryology, and molecular phylogeny need to be used to reach an agreement. No one approach alone produces satisfactory products.
I had given a review of somitomeres at an international meeting in Chicago also attended by R.P.S. Jefferies from the paleontology department at the Natural History Museum in London. This chance meeting lead to several years of interesting interactions and collaboration. Jefferies had studied, for over 30 years, a group of fossil animals that he termed 'mitrates'. They had an echinoderm-like calcite skeleton, but some also had characteristics, such as gill slits, associated with chordates. I visited the London Natural History Museum to see Jefferies' fossils for myself, and he visited me in Austin, Texas. We decided to reconstruct the latest common ancestor of extant craniates, and to suggest how this animal x arose from the fossils known as mitrates. More concretely, we aimed to show how an animal somewhat resembling a hagfish arose from one much like a giant calcite-plated tunicate tadpole.
We proposed how the anatomical transitions from mitrate to animal x might have been produced by changes in embryology. For a long time, most people assumed that the sister group of the craniates was the lancelets. Jefferies was one of the few to maintain that the sister group was the tunicates, based on his fossil studies. In more recent times molecular phylogeny people first confirmed the lancelets as the sister group, then later additional molecular studies using more primitive genes concluded that the sister group of the craniates is the tunicates.
We published our studies in 1998 in Integrative Biology 1(4): pp.115-132, An Episode in the Ancestry of the Vertebrates: From Mitrate to Crown-Group Craniate. by R.P.S. Jefferies and A.G. Jacobson. The journal did not persist, but a pdf of the paper is here.
In a paper in Nature in 2002 (Dominguez, P., Jacobson, A. G. & Jefferies, R. P. S. 2002. Paired gill slits in a fossil with a calcite skeleton. Nature (London) 417:841-844.), pdf here, we analysed a mitrate with both a calcite skeleton and paired gills.
Dr. Jefferies' statement about this paper is at the DigiMorph site.