In addition, many enantiornithines retain teeth in their mandible and claws on their forelimbs, features that are omnipresent in dinosaurs but were subsequently lost on the evolutionary path towards modern birds (although hand claws are rather selectively lost, since many living birds still have hand claws).
It may indicate that in one clade the characters in question were never present, providing evidence against a close evolutionary relationship with clades that possess them, or it could be that the characters were originally inherited by both clades from their common ancestor but were subsequently lost in one.
The introns-early hypothesis states that spliceosomal introns were present in the last common ancestor of prokaryotes and eukaryotes but were subsequently lost in all prokaryotes [ 4].
Such elements were likely present in some lineages early in animal mitochondrial genome evolution but were subsequently lost during the reduction of intergenic regions, which occurred in the Eumetazoa lineage after the split of Porifera.
If we assume, however, that Archaea is a derived lineage from gram-positive bacteria [ 36, 37], then HTPG proteins originated before the origin of Archaea, but were subsequently lost in this lineage.
Others have representatives in only a few species, suggesting that they either arose via a later duplication or that they were present in the LCA of Teleostei but were subsequently lost in some lineages.
Our results strongly suggest that myod was duplicated during the basal teleost whole genome duplication event, but was subsequently lost in the Ostariophysi (zebrafish) and Protacanthopterygii lineages.
In one, rope-building appeared in the common ancestor of all filamentous cyanobacteria, but was subsequently lost in one or more lineages over time as they evolved and adapted to different habitats.
However, it seems possible that a progenitor Class III eIF4E-gene evolved early in metazoan evolution, but was subsequently lost in some groups.
