Tendons mediate muscular-skeletal junctions in vertebrates and are key to movement and stabilization of muscles. The evolutionary origins of tendons are unclear, as they are absent in invertebrate chordates and are thus vertebrate novelties. We chose to look into the developmental origins of tendons in cyclostomes, specifically sea lamprey, an understudied vertebrate group which diverged from other vertebrates more than 500 million years ago. It is known that lamprey have true muscles and skeleton based on marker gene expression data and histochemical studies. We hypothesize, therefore, that tendons are a fundamental structure to the vertebrate musculoskeletal system and that this cell type existed in the last common ancestor of lamprey and gnathostomes. To test this, we investigated tendon development in embryonic sea lamprey Petromyzon marinus. We selected seven different genes that are known to be factors in tendon development in gnathostomes (Psx, Mkx, Xirp2, Egr1, Col3a1, Col1a2, Col12a1). We used in situ hybridization paired with sectioning and histochemical techniques to analyze when and where these tendon marker genes are expressed in the sea lamprey embryo. Our results found that tendon marker genes are spatially and temporally distributed throughout the embryonic lamprey body- expressed in the myosepta, myomeres, facial muscles, pharyngeal muscles, and pharyngeal skeleton at various stages in development and in both muscle and skeletal cell types. We note the presence of multiple tendon marker genes in a muscle thought to be unique to lamprey: the hypobranchial muscle. We discuss implications of this spatial, temporal, and cell-type mixture of tendon marker gene expression for vertebrate musculoskeletal evolution.