Itâs not quite a chicken-and-egg paradox, but scientists have long wondered which came first, teeth or scales. A new analysis of some toothlike fossils suggests the answer may be scales, contradicting decades of conventional wisdom.  The idea that teeth came first was based largely on 500-million-year-old fossil fragments known as conodonts. They look like teeth or spines, and contain tissue that resembles dentin and enamel, strongly implying they were an early precursor to teeth in vertebrates â appearing long before the emergence of scales.  Now, using high-resolution X-rays to study conodonts from several centuries, a team of paleontologists has determined that they are not directly related to vertebrate teeth, potentially upending the teeth-first theory.  âIf you look at ancestors of conodonts, what you see is gradual assembly of toothlike characteristicsâ rather than a direct evolutionary progression, said an author of the study, Philip C. J. Donoghue, a paleobiologist at the University of Bristol in England. The researchers, who published their findings in Nature, also reported that the conodonts did not contain dentin, âso the whole hypothesis collapses,â Dr. Donoghue said.  Teeth and scales are made of related tissue. Thus a scenario in which teeth came first suggests an âinside-outâ theory of evolution, in which the tissue originated in the mouth, then grew across the outside of the body in the form of scales. In that model, scales were considered a predatory (rather than defensive) attribute.  Now Dr. Donoghue and his colleagues suggest that inside-out be replaced by outside-in. Scales were most likely intended for âprotection rather than predation,â he said. âAn extremely elaborate armor.â
Scientists have learned that, contrary to decades of conventional wisdom, scales may have come before teeth in vertebrates.