However, there are issues with positing "Tiktaalik" as a tetrapod ancestor. For example, it had a long spine with far more vertebrae than any known tetrapod or other tetrapodomorph fish. Also the oldest tetrapod trace fossils (tracks and trackways) predate it by a considerable margin. Several hypotheses have been proposed to explain this date discrepancy: 1) The nearest common ancestor of tetrapods and "Tiktaalik" dates to the Early Devonian. By this hypothesis, the lineage is the closest to tetrapods, but "Tiktaalik" itself was a late-surviving relic. 2) "Tiktaalik" represents a case of parallel evolution. 3) Tetrapods evolved more than once.