The Largest Trilobites

    The size variance shown among trilobite species is nothing less than astonishing. Generally when one considers these long-extinct arthropods, images of small, hard-shelled creatures scurrying across ancient sea floors spring readily to mind. And for the most part, such a perception is accurate. The vast majority of the more than 25,000 scientifically recognized species that existed during the trilobites' 270 million year reign from the Lower Cambrian to the end of the Permian were diminutive -- an inch, or smaller in length. By no means, however, should such information be interpreted as indicating that trilobites didn't grow bigger than that… much bigger! 
    Perhaps the most notable of these large trilobite species are the Ordovician-age Isotelus maximus from the mid-western United States, a type that grew up to 16 inches in length and became recognized as the State Fossil of Ohio back in 1985. In addition, impressively sized Silurian-age trilobites have been found in western New York State, with species such as Trimerus delphinocephalus attaining lengths up to eight inches.  And a variety of species, including Hungoides bohemicus, that on rare occasion exceed 12 inches from head to tail, have been found in the prolific Ordovician quarries near Volongo, Portugal, marking the entire formation as one filled with mega-fauna. 
      Indeed, over the last century, partial or badly disarticulated specimens from around the world have provided tantalizing evidence of the great size that some trilobites might have attained. Some, such as with the legendary, spinose Devonian lichid, Terataspis grandis, may have grown to over two feet in length. Yet complete examples of such extraordinary discoveries had been few and far between… at least before the fossil “revolution” that has occurred world-wide over the last 30 years.
    First, the now-storied trilobite beds of Morocco started to produce Cambrian-age Paradoxides trilobites of prodigious size… and in equally prodigious numbers. By the late '80s, thousands of specimens, some 18 inches long, began to emerge on the world stage, highlighting museum displays, flooding fossil trade shows and even appearing in local rock shops. These massive specimens almost single-handedly changed public perceptions about trilobites; no longer were they fingernail-sized fossils designed for well-manicured cabinets of curiosities or carefully curated institutional shelves. Now they were often presented as natural monstrosities, remnants of the primordial past that could be viewed from across the room… even if that room happened to be a grand gallery filled with hundreds of invited guests.
     Only a few years after these giant Moroccan trilobites started hitting the world market, dozens of 10 inch examples of the Ordovician Dikelocephalid trilobite Asaphopsoides brevica began emerging from central China. And in 1998, a 28 inch-long specimen of Isotelus rex (roughly “The King of Trilobites”) was being collected by members of the Manitoba Museum and University of Manitoba field party along the shores of Hudson Bay in Manitoba, Canada, making it the largest complete trilobite yet found. 
      An obvious question then began to emerge: why did trilobites in certain locations grow to such astounding dimensions? One of the current hypotheses speculates that gigantism can be brought on as an adaption to a colder aquatic climate. Other scholars have speculated that much like some modern-day arthropods, trilobites might have continued growing throughout their lives, molting their hard exoskeletons on a regular basis to reflect their ever-increasing dimensions. 
     It is apparent that many key questions concerning trilobite growth remain to be answered. But it seems safe to say that judging by the recent stream of “prehistoric monsters” that have emerged around the globe, science's perception of trilobites as diminutive examples of ancestral life will never be quite the same again.

Here is a look at some of the largest trilobites in the world:

Acadoparadoxides levi-settii: These Cambrian trilobites from Morocco, which reached lengths up to 18 inches, have been found in prodigious numbers.

Asaphopsoides brevica: A rare Ordovician species from China that often attained lengths of 10 inches.

Uralichas hispanicus tardus: One of the largest trilobite species, with Moroccan examples frequently exceeding 12 inches.

Image courtesy of Manitoba Museum 

Isotelus rex: The “king”… this 28 inch specimen from Churchill, Manitoba holds the current record for trilobite size.

Isotelus latus: Another large Canadian asaphid, this Ordovician species routinely reached lengths of 10 inches.

Trimerus delphinocephalus: This strange-looking trilobite from the Silurian of New York could attain lengths of 8 inches.

Hungioides bohemicus arouquensis: From the renowned Volongo formation of Portugal, this species was representative of a maga-fauna which produced a variety of large trilobites, often 12 inches is size. 

Asaphus sp.: A layer of these giant Ordovician trilobites-- frequently 20 inches long-- were found in Morocco during the early years of the 21st Century.