So what was made of this surprisingly high profile case? Sir Richard Owen treated the questionable illustration like a photograph and concluded that the head shape and reference to a mane indicated a mammal and proposed that it was a Southern Elephant Seal (Miroungna leonina)*. Cryptozoologists often take reported sizes far too seriously and it would not be surprising for even experienced mariners to inflate an unfamiliar 5-6 m pinniped into a ~18 m monster taking the wake and lack of scale into account. Owen's case runs into trouble since male elephant seals are typically on rookeries at that time of the year, they lack any remotely mane-like structure**, their profile should be instantly recognizable***, they most certainly do not have a stiff body as Owen claims, the reported locomotion sounds very unlikely for a phocid, and the captain of the Daedalus reaffirmed that the head he saw was flat****. Since there aren't many other large animals in the area which can swim with their heads above water, Owen was right to consider elephant seals as a candidate for the Daedalus sighting; however the lack of other candidates considered, his adamant support despite numerous confounding factors, and the fact that he apparently recently saw a juvenile in captivity leads me to think that this was a mild case of Phylogenetic Roulette.
* Despite the fact that he proposes Phoca proboscidea, Phoca leonina, and "Anson's sea lion" as candidates, these all appear to be synonyms for M. leonina (King and Bryden 1992). His letter can be read here, page 280 or so.
**If the animal was in poor condition, its protruding spine could theoretically form a "crest" of sorts. Owen implied that his hypothetical elephant seal was in poor condition because he erroneously thought they couldn't spend more than a day at sea - they can stay at sea for months.
*** This could be hand-waved if the hypothetical seal was immature. The reported overbite could possibly be taken as evidence for a developing proboscis.
**** Heuvelmans gives the illustrations more credibility and concludes that the animal may have been an unknown species of plesiosaur-like pinniped. Despite claiming the opposite, Heuvelmans could be a painfully biased pigeonholer.
While Owen had at least some reasoning for selecting an elephant seal as a candidate, Heuvelmans mentions a severe case of Phylogenetic Roulette: one anonymous correspondent suggested the eel-like fishes (or highly derived true eels) Saccopharynx flagellum or "Ophiognathus" ampullaceus (now Saccopharynx ampullaceus) as candidates. Heuvelmans summarizes this very well:
This lands squarely on the non sequitur side of the spectrum. Of all the lineages of elongated or serpentine oceanic organisms, what could possibly set gulper eels apart from the hundreds of other candidates? It boggles the mind how fish with morphology so completely at odds with what was reported could even be selected as candidates, yet this happens all the time in cryptozoology.
[the] suggestion... might seem very learned to the common reader until he discovers that these impressive Latin names refer to two closely related abyssal fish, eel-like in shape, but with vast mouths capable of swallowing four times their own weight, and never, so far as we know, exceeding 6 feet in length. The suggestions must have come from an amateur impressed by the strangeness of these fish rather than an experienced zoologist, for they could never be mistaken for the Daedalus sea-serpent.
Daedalus cryptid - giant squid! A cryptid in Lake Champlain - derived Tanystropheus! A carcass from the stomach of a sperm whale - extant Sauropterygian! Raccoon carcass - sea turtle without a shell (sic)! Who knows how many thousands of half-baked identifications have been proposed on cryptozoology forums - is there some wheel of big, bizarre animals that these people spin whenever a report or ambiguous carcass crops up? Making an identification from the general shape of a reported cryptid is far from scientific - you need specific morphological characters like those kindly put up on the Palaeos page to establish an objective case. Phylogenetic Roulette probably stems in part from our brains being hardwired to see patterns and answers with limited data; while this can be useful in real life things can be much more ambiguous, there are few easy answers. I'd hold up Paxton et al. (2005) as an objective and ideal analysis of a purported cryptid report, hopefully more like it will follow.
There is no way to stop quackery and most dumb suggestions, but if done properly I'd say cryptozoology does show some promise.
Heuvelmans, Bernard. 1968. In the Wake of the Sea-Serpents. Hill and Wang, New York.
King, J. K. and Bryden, M. M. 1992. Mirounga leonina. Mammalian Species 391, 1-8
Paxton, C. G. M. et al. 2005. Cetaceans, sex and sea serpents: an analysis of the Egede accounts of a “most dreadful monster” seen off the coast of Greenland in 1734. Archives of natural history 32, 1-9