The anchor-and-dolphin design shown on the side of the baptismal font and embedded in the mosaic floor of the old baptistery (today’s Adoration Chapel), is a surprisingly complicated symbol.
The two parts of the design are often read as two separate pictures, then combined. The 1960 SFDS Parish Monthly Bulletin offered a typical explanation of the anchor, connecting it with water and hope: “Hope was represented…by the anchor which the sailor drops into the water, so that it may go down deep into the bottom of the sea and fix itself firmly in order to steady the ship and hold it secure against the winds and waves of any storm. But our hope is an anchor which we throw upward, into the skies of heaven…” Philip Kosloski, at Aleteia, more recently discussed the dolphin symbol, observing that in the ancient world, dolphins “were known as the ‘sailor’s friend’ and there are many legends of dolphins leading mariners to safer shores…” He suggested that over time, “dolphins became a symbol of Jesus Christ, a friend and deliverer to the ‘safer shores’ of heaven.” He then addressed the combined symbol of “dolphins… twisted around an anchor or trident…” which symbolize” the hope of eternal life…”
Rather than a picture symbol, the original meaning could actually have been language-based. Charles Kennedy puzzled long ago in Biblical Archeology Review, that anchor designs were common on graves in Christian catacombs until the third century, but then they disappeared. Around the same time, the main language of Christians switched from Greek to Latin. Kennedy suggested that “Ankura,” (Greek for “anchor”), could have been a pun on the Greek phrase “en kurio” (“in the Lord”) — so that with the symbol of an anchor, “the dead are sealed with the name of the Lord.” When the language changed, the pun didn’t work anymore and the symbol was abandoned for a time.
As to dolphins – Aristotle called them “fishes,” and his basic classification scheme remained in use in Europe until the 1800s. ICTHYS, Greek for fish, was used in catacombs as an acronym for the Greek phrase “Jesus Christ, Son of God, Savior.” So, a fish on an anchor in the catacombs could have been a simple linguistic symbol meaning that someone was protected “In the Lord Jesus Christ, Son of God, Savior.”
Whatever its origin, the combined symbol of the dolphin and anchor was not much used until the Renaissance, when printer Aldus Manutius, who printed books for the influential Medici popes, adopted a dolphin-and-anchor representation of Neptune as his printer’s emblem – but that’s a whole ‘nother story for another day!