One of the best-known groups of fossil animal are the trilobites ('three-lobed'), this name deriving from the distinctive three-lobed structure of their body, which consists of the cephalon (head shield), the thorax, and the pygidium (tail shield). Ranging in size from a dinner plate down to a pea, they are famed for their segmented body form, numerous pairs of limbs, and extremely well-developed compound eyes.
Global but exclusively marine in distribution, this taxonomic class of arthropods was one of the earliest, with the first-known representatives in the fossil record dating back approximately 540-520 million years to the early Cambrian Period (though it is suspected that there may well have been earlier forms as yet unrepresented by documented fossils dating as far back as 700 million years, to the pre-Cambrian).
Beautiful illustration of trilobites from Système Silurien du Centre de la Bohême, by Joachim Barrande, 1852 (public domain)
Bearing in mind how zoologically familiar they are today and how their taxonomic identity as arthropods is indisputable, it may come as something of a surprise to learn that the first trilobite fossils to attract notable scientific attention, during the 1700s, incited considerable controversy as to what type of creature they represented, resulting in some exceedingly bizarre notions being aired in all seriousness.
They were initially deemed to be ancient, three-lobed clam-like seashells (and were duly dubbed Concha triloba), because these particular trilobite fossils showed only the animals' dorsal side (thereby concealing the fact that trilobites actually possessed legs – lots of legs, in fact!).
Sentimentally priceless - a diverse selection of trilobite casts on a matrix slab bought for me by my late mother Mary Shuker during our first, very happy visit to Lyme Regis, Dorset (© Dr Karl Shuker)
Not everyone agreed with this identification, however, and offered various alternative but equally erroneous suggestions. The normally authoritative British zoologist Dr George Shaw (1751-1813), for instance, proposed that trilobites were fossil caterpillars, while some contemporaries opined that they were archaic centipedes, or (less preposterous) crustaceans.
The matter remained contentious until American palaeontologist Charles D. Walcott resolved it in a very convincing manner – by skilfully and painstakingly using a hacksaw to open up no fewer than 3,500 fossils of curled-up trilobites, thereby revealing the presence of their jointed legs, and, in turn, these hitherto-baffling beasts' true nature as arthropods.
Spectacular, life-like model of Bristolia bristolensis, a notably long-spined species of early Cambrian trilobite from the southwestern USA (© Andrew 'Trilobite' Scott – click here for a ShukerNature exclusive showcasing many other examples of Andrew's stunning artwork)
During the lengthy course of their evolution, the trilobites became exceedingly successful, yielding a vast diversity of species (some 17,000 are currently recognised) as well as body forms and lifestyles before decreasing markedly in the Devonian, and finally dying out completely around 252 million years ago (in the mass extinction that occurred at the end of the Permian) – or did they? There is no well-established reason why they should have done.
As a zoologist living in the West Midlands, England, I am very aware that one particular trilobite species, Calymene blumenbachii from the Silurian Period, is so abundant in the fossiliferous limestone quarries of Wren's Nest in the West Midlands town of Dudley that it is popularly known as the Dudley bug or Dudley locust, and even appears on the Dudley County Borough Council's official coat-of-arms. Naturally, therefore, I've been a fan of trilobites ever since childhood, and my fossil collection contains several specimens, but my interest in cryptozoology would subsequently yield an additional reason for my being fascinated by them.
In the mid-1980s, I purchased veteran American cryptozoologist Prof. Roy P. Mackal's classic book Searching For Hidden Animals (which had originally been published in 1980 in the U.S.A., but not until 1983 in the U.K.), and was delighted to find that it documented a wide range of lesser-known cryptids.
However, one chapter that obviously attracted my particular interest was tantalisingly entitled 'Living Trilobites?'. It included a discussion as to whether any representatives of these archaic arthropods might have survived the Permian mega-death and persisted in benthic anonymity on the ocean floor into the present day.
Prof. Roy P. Mackal and the UK hb 1st edition of his book Searching For Hidden Animals (© Prof. Roy P. Mackal/Cadogan Books)
As Mackal noted, many trilobites were shallow coastal dwellers (especially the later ones), yet no living trilobites from such localities have ever been discovered. Consequently, the only hope for modern-day survival is if "some forms adapted to a deeper, more obscure environment and there found refuge" - or if some that were already so adapted simply persisted. Should this scenario have indeed taken place, it could explain why no Cenozoic trilobite fossils have ever been found - because these would not be readily discovered or accessed on the ocean floor. But what about obtaining living specimens there?
Until reading Mackal's book, I hadn't been aware that the very first global marine research expedition, the voyage of HMS Challenger from 21 December 1872 to 26 May 1876, seriously believed that living trilobites might be dredged up from the ocean bottom. But although countless specimens that included representatives of over 4,000 hitherto-unknown animal species were indeed procured there, none of them were trilobites. Or, as worded in the authoritative Encyclopaedia Britannica's eleventh edition, published in 1911, the "faint hope" of finding such creatures was not realised.
In reality, however, a few years before this expedition had even set out on its epic voyage of zoological discovery, a claim had been made that a living trilobite had already been obtained, and from a depth of 1,200 fathoms (7,200 ft). Moreover, this claim was actually believed for a time before the creature's true, non-trilobite identity was revealed. Another veteran cryptozoological chronicler, Willy Ley, who briefly reported the case in one of his many articles, didn't provide further details, but as noted by Mackal the timing and morphological similarities strongly suggests that the discovery in question was actually that of a certain Antarctic species of isopod crustacean (the taxonomic group which includes woodlice and sea slaters) that is astonishingly trilobite-like in outward appearance.
Brought to scientific attention in 1830, its first officially recorded specimen had actually been found inside the gut of a marine fish examined by American naturalist Dr James Eights while visiting the South Shetland Islands between Patagonia and Antarctica during the so-called 'Expedition of 1830'. Emphasising its remarkable morphological convergence, in 1833 Eights formally christened this memorable new species Brongniartia [now Ceratoserolis] trilobitoides. And it was indeed initially mistaken for one of these prehistoric arthropods by some observers, but it sports two pairs of antennae (a crustacean characteristic), whereas trilobites only had one.
Other modern-day creatures that have often been mistaken for living trilobites are chitons and water pennies. Chitons (or polyplacophorans, to give them their formal zoological name) constitute a taxonomic class of molluscs characterised by their very distinctive shells, which are composed of eight separate but slightly overlapping plates, and afford these animals a superficially segmented appearance dorsally.
If a chiton is turned over, however, its ventral body surface is seen to be non-segmented and only possessing a single, typically-molluscan foot, in contrast to the many limb pairs possessed by trilobites. As for water pennies, these trilobite imposters are the larval stage of certain aquatic freshwater psephenid beetles, belonging to the genus Mataeopsephus.
Also superficially trilobite-like in outward dorsal appearance are both the larvae and the larval form-retaining adult females of lycid (net-winged) beetles belonging to the genus Platerodrilus, and which are therefore known colloquially as trilobite beetles (click here for a ShukerNature article featuring these distinctive insects). Native principally to tropical rainforests in India and southeastern Asia, some of them are brightly coloured.
Finally, the juvenile stage of those famous 'living fossils' known as xiphosurans or horseshoe crabs is termed a trilobite larva, once again because of its superficial similarity to genuine trilobites. Horseshoe crabs, incidentally, are the closest living relatives of another taxonomic group of iconic fossil arthropods – the eurypterids or sea scorpions.
Exquisite illustration by Ernst Haeckel from 1904 featuring trilobites, horseshoe crabs, and sea scorpions (© public domain)
Back in the 1980s, a bizarre story emanating from Australia briefly hit the news headlines, claiming that some trilobites had been found inhabiting Perth's storm drains. Not surprisingly, however, this was soon exposed as a hoax, featuring an old tyre that had been cut into the shape of a trilobite.
Numerous deepsea collecting expeditions have been launched since Challenger, but none has ever procured any living trilobites, and yet some tantalising indirect (or, to be precise, ichnological) evidence for such creatures may have been recorded, which Mackal described as follows:
...in 1967, I was invited by Ralph Buchsbaum, professor of zoology at the University of Pittsburgh, to give a seminar on our researches at Loch Ness. During the social hour after the presentation one of his colleagues told me about experimental photography of the sea bottom that was in progress. He stated that photographs of fresh tracks identical to the Cruciana [sic – Cruziana], the fossilized trilobite tracks, had been obtained. He expressed the hope that traps could be lowered to catch whatever was making these highly suggestive tracks. As far as I know the nature of these tracks was never determined and nothing was ever trapped, because of a subsequent loss of funding for the project. The business of identifying sea-bottom trails and tracks is a tricky one and to infer living trilobites from a track is even more tricky. A marvelous collection of sea-bottom tracks and trails is presented in a book entitled The Face of the Deep by B. C. Heezen and C. D. Hollister. Only a tiny fraction of aquatic animal tracks have been identified, so that fertile ground for new discoveries is indeed abundant...Underwater photography of the ocean floor...appears to be a promising tool for future cryptozoological expeditions.
Cruziana is a famous trace fossil taking the form of elongate, bilobed burrows that are roughly bilaterally symmetrical. As noted in a 2010 Lethaia paper by Dr Stephen Donovan, many examples are believed to be the tracks or trails yielded by trilobites while deposit-feeding, but certain others are deemed not to be, because they were present in freshwater environments (where no trilobite fossils have so far been found) and/or were of Triassic date, by which time all trilobites were supposed to have died out. But were these ostensibly anachronistic tracks actually made by surviving post-Permian trilobites for which direct fossil evidence has simply not been found as yet?
Incidentally, two other types of trace fossil believed to have been created by trilobites are Rusophycus and Diplichnites. The former fossils are excavations featuring little or no forward movements, and have therefore been interpreted as traces left by trilobites while resting or in defence/protection mode. In contrast, the latter fossils are believed to be traces left by trilobites while walking upon the sediment surface.
Mackal ended his living trilobites chapter on a somewhat pessimistic note, concluding: "While not impossible, it is most improbable that living trilobites still exist". After that, this fascinating prospect appeared to have vanished from the modern world just as surely, it would seem, as the trilobites themselves – which is why I was so startled, but delighted, by a certain comment allegedly made by a well-respected current scientist more recently.
On 24 June 2004, Yahoo! News released online a report concerning the receipt of a $600,000 start-up grant from the private Alfred Sloan foundation for a proposed 10-year international survey of the oceans' depths, at an estimated total cost of US $1 billion, to be funded by governments, companies and private donors, and officially dubbed the Census of Marine Life (CoML). As part of this grand-scale project, scientists led by researchers from the University of Alaska planned to use robot submarines and sonar to track down life forms in the Arctic Ocean's chilling deepwater domain, and expectations were that by the end of its decade-long course, the survey could easily have doubled the number of species known from this particular ocean.
Three-dimensional reconstruction of Drotops armatus, a very spiny species of Devonian trilobite from Morocco (public domain)
All very worthy indeed, but what caught my eye amid all of these statements was one attributed in the news report to none other than Dr Ron O'Dor, chief scientist of the multi-nation CoML. According to the report, whose exact wording is quoted here as follows, Dr O'Dor "speculated that Arctic waters might hide creatures known only from fossils, such as trilobites that flourished 300 million years ago". It would seem, therefore, that the notion of finding living trilobites has not been entirely discounted by scientists after all.
Happily, the CoML did indeed take place, this very ambitious project ultimately featuring scientists from more than 80 different nations, and releasing the world's first-ever census in 2010 – but no living trilobites were listed. Nevertheless, there is a notable precedent well worth mentioning here.
The monoplacophorans are a primitive taxonomic class of molluscs, whose youngest fossil species date from around 380 million years ago. On 6 May 1952, however, trawling off Mexico's western coast at a depth of almost 12,000 ft in dark, muddy clay, the Danish research ship Galathea hauled up 10 complete specimens and three empty shells of a small, seemingly unremarkable mollusc superficially resembling a limpet but which proved upon scientific examination to be a living monoplacophoran. This hitherto-unknown species was formally named Neopilina galatheae, since when further specimens of it, and of several additional modern-day species too, have been obtained (see my Encyclopaedia of New and Rediscovered Animals, 2012, for full details).
Structurally, these living monoplacophorans are very different internally from their archaic fossil ancestors, so if living trilobites do exist, these too are likely to be highly evolved species. Nevertheless, the discovery of Neopilina and kin readily demonstrates that it is by no means impossible for invertebrates deemed by their fossil record to have died out in very far-distant prehistoric ages to be represented, in fact, by living species that have simply evaded scientific detection.
This ShukerNature blog article is adapted from my forthcoming book Still In Search of Prehistoric Survivors.