The sea turtle: a skeleton turned inside out

This 17th century print of a turtle’s skeleton by Andries Jacobsz. Stock, is part of a series of animal skeletons after prints by the Italian print maker Teodoro Filippo di Liagno (c.1587-1630). Di Liagno, who worked at the court of Cosimo II De’Medici in Florence, produced this series of etchings of animal skeletons for Johann Faber, a medic and botanist who in 1600 became a professor of Botany and Anatomy at the Sapienza University in Rome.

Skeleton of a Turtle, Andries Jacobsz. Stock. Etching from a series of anaimal skeletons by printmakers Hendrick Hondius I, Andries Jacobsz. Stock and Simon Frisius, after Teodoro Filippo di Liagno,1625 – 1626, etching, 98mm × 155mm (RP-P-1997-289)

Turtles have a truly extraordinary anatomy that fascinated researchers around the world in the past up till this day. We will explore the peculiar anatomy of the sea turtle, which will prove to be one of the most unique on this planet. The fascination with the extraordinary anatomy of the turtle goes back many centuries. Conrad Gessner’s work on zoology, Historiae animalium (published in Zürich, 1551–1587), one of the earliest texts on modern zoology and contains a beautiful image of the Testudine Marina.

Conradi Gesneri medici Tigurini Historiae animalium lib.I-V, In five volumes, each with a separate dated title page. 1551, 1554, 1555, 1587 respectively.

Expeditions to the New World led to the discovery of strange and exotic fauna which led to early modern natural scientists to change the existing system of animal classification. One of the new scientific methods of the early modern period for zoology was the study of animal anatomy, through dissecting animals as well as closely studying taxidermized examples. The invention of the printing press allowed for the reproduction and easy dissemination of this newly found anatomical knowledge. There are interesting examples of anatomical drawings and prints depicting the turtle’s anatomy such as this print by Jan Luyken, from  Osteologia, published in Amsterdam in in 1680, and clearly based on Di Liagno’s series, or its Dutch reproductions. In van Dyck’s treatise, we also find the table corresponding to the different letters on the print, which explains how many bones each of the parts of the turtle has. Though nowadays these numbers aren’t considered accurate anymore, the print reflects the vivid interest in anatomy in the early modern period.  

Skeleton of a Turtle, Jan Luyken, after Cornelis van Dyk, 1680, etching, height 86mm x width 139 mm, (RP-P-OB-44.113). With accompanying legend. Source: Cornelis van Dyk. Osteologia, of Nauwkeurige geraamt beschryving van verscheyde dieren, nevens hare historien, Volume 1. Amsterdam,1680. p. 147-8.
Osteologia, of Naukeurige geraamt beschryving van verscheyde dieren, nevens hare historien, by Cornelius van Dyk, Apothecer & c tot Medemblik, published in Amsterdam in in 1680

A still existing common myth about the turtle and its shell, is that the two are separable. Unlike other shelled animals that must trade or shed their shells as they grow, the turtle’s shell is a permanent part of its body, growing with it as the turtle gets older. The shell in fact is the turtle’s skeleton, which, as we have seen in the prints above, was clearly understood in the 17th century. We will discuss this remarkable text and Di Liagno’s remarkable print series in more detail in a future blog.

The turtle is one of the oldest reptiles with 250 million-year-old fossils that predate the dinosaurs. These first examples have no external shield yet but do display broadened ribs. The oldest fossil evidence of a full shell dates back some 210 million years and displays how the ribs evolved to broaden and fuse into a shell. A famous example is the fossilized Archelon (Yale Peabody Museum), an extinct giant sea turtle, which was found in North American rocks, dating 100 to 66 million years ago, and has a shell that is similar to the carapace from modern sea turtles.

Frederic A. Lucas – “Animals of the past”, 1903.

The bony shell of sea turtles consists of two halves: the upper carapace and the lower plastron. The plastron consists of several bone plates, while the carapace consists of broadened ribs that are merged with the vertebrae on the inside. What we observe when we see a turtle are not its bones, but instead a layer of keratinous patches called scutes. It is these scutes with their wonderful patterns that have been used in large quantities as a decorative material, which will be discussed in a future blog.

Returning to the shield in the collection of the Rijksmuseum, we know that it consists only of the upper part of the turtle’s shell: the carapace. On the inside we clearly see the structure of the broadened ribs, and remains of the fused vertebrae. Underneath the paint layers the scutes are still in place, as was confirmed by an X-ray image. As all turtle species have a specific amount and arrangement of scutes, we were able to identify the species to which the shield belonged. As it turns out, the shield comes from a Chelonia Mydas, the green sea turtle.

The green sea turtle’s territory spans over the entire world, with reported encounters across all continents. Among the seven marine turtles, the green turtle is the only herbivore. This could be an explanation for its reportedly tender meat, which explains its colloquial name soup turtle (expect a future post about the culinary aspects of this turtle).

Nowadays, six out of seven marine turtles are listed on the IUCN red list of threatened species, with our green sea turtle as an “endangered” species. While thousands of hatchlings are born every year, their odds of survival are estimated at just 1 in 1,000 to 10,000. This is in part caused by natural threats such as predators. However, the shift towards extinction is a recent one caused by various anthropogenic factors such as pollution, illegal harvesting and trade, climate change, and habitat loss. Already in 1671 there was a call for the formulation of a “bill for the preservation of a turtle”. Unfortunately this seems to have been a call in vain, as the numbers are dropping quicker than ever today.

Source: James J. Parson, The Green turtle and man, (Gainesville: University of Florida Press, 1962),24.

Ever since the world has gone into lockdown, nature has taken the chance to flourish once again in places that usually would be overflowing with people and their waste. We have seen mountain goats take over empty Welsh streets, and marine wildlife return to the now tranquil waters in Venice. One of the species that seems to be profiting most from the sudden withdrawal of mankind are endangered sea turtles, whose nesting places on beaches would normally be threatened by crowds of tourists. Now that these beaches are empty, record numbers of nesting places are being reported all over the world, from Brazil to Florida and Thailand.

Sea Turtles on the beach in Maui, Hawaii. Photo by Jolo Diaz. 

Next week’s blog will focus on turtle trade, with fascinating entries from pirate’s diaries! To end on a positive note, let’s marvel once more at the beauty of this remarkable reptile and learn that it is not impossible to turn the odds in favour of their survival, as shown by the – small, but significant – recent thriving of the turtles!

The Turtle Team

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