Notes on Objects: Longhorn Beetle and Ammonite

I volunteer as an object-handler at Manchester Museum (I am there on Wednesdays from 11am to 1 pm, at the table near Nature's Library), which involves inviting people to handle certain objects from the museum's archives and talking to them about it. I often have a taxidermied fox, which is a good hook to bring people in, and they often have the slightly philosophical question, "Is it real?". It is real fox fur (and maybe claws?) but the insides and eyes aren't real, the insides are a polyurethane mold and the eyes are plastic. So it is a semi-real fox. Many people are very surprised that it feels so soft around the ears, a bit like a dog.
But my two favourite objects are the Giant African Longhorn Beetle and an unidentified ammonite, and I will give some further information about them here.

Giant African Longhorn Beetle (Petrognatha gigas)

This is a insect of the family Cerambycidae (the long-horn beetles), and the "horns" of it's name are it's antennae, which are used in insects for sensory purposes, including orientation, detecting sound and sensing chemicals. The Giant African Longhorn Beetle lives in dead acacia trees, and it is well suited to this, as it's long antennae and limbs look like the twigs of the acacia tree. The spikes on it's thorax (the "neck") and top of the abdomen look very like the prickles of the acacia tree.

Beetles are insects with “sheathed wings” according to the literal translation of their scientific name Coleoptera. All insects have two pairs of wings, the hardened forewing (elytra) and the hindwing, which is a fairly standard insect wing. The elytra protects the hindwing in flying species. In ground beetles, the elytra are fused together, therefore making them flightless and confined to the ground. All beetles have a hardened exoskeleton, which is made of sclerite plates separated by a rigid joint (suture) which provides both armour and flexibility. Not a lot is known about Giant African Longhorn Beetles as they have not been studied much, though they do have a particular way of breathing known as Bernoulli suction ventilation, see this article to learn more. 

  Cerambycidae is a large family of over 20,000 species. The largest member of this family is the Titan beetle (Titanus giganteus), which has a maximum body length of 16.7 cm. The longest beetle of all is the Hercules beetle (Dynastes hercules) which is up to 17.5 cm long and can lift up to 8kg over 850 times their own weight. Hercules beetles are rhinoceros beetles, so named because the male has two large horns on it's head, adding to it's length, which are used for fighting.  The larvae of this family are called roundhead borers, as they bore into living or recently felled wood, and are considered a pest by the logging industry, though the larva of the cactus longhorn beetle bore into prickly pears and chollas.

There are around 400,000 known species of beetles (25% of all know animal species), and there are probably over 1 million total known and unknown beetles in the world. 

Ammonite (of unknown classification)

 Ammonites were shelled, marine cephalopods (the same group which octopuses and squid belong to) which existed from the mid Devonian (400 million years ago) to the late Cretaceous (66 million years ago), in the KT (Cretaceous-Tertiary or Cretaceous- Paleocene) Mass Extinction which was probably caused by the effects of an asteroid impact and also caused the extinct of the non-avian dinosaurs. The ammonite I have on my handling table  is 160 million years old, placing it in the Late Jurassic (in the Oxfordian age), which is 95 million years older than T. rex. Ammonites as a group are much older than the oldest dinosaur (probably Eoraptor, which is 231.4 million years old), and even older than four-legged animals (Tetrapods) which evolved after 395 million years ago.  The white stuff in the sections of the shell is cacite crystals, which were formed when the shell was being fossilised. Water with dissolved salts in it got into some of the outermost sections of the shell, and as the shell was being compressed under the weight of sediment on the ocean floor, the water evapourated and left behind the salts, which grew into crystals. 

They are named ammonites due to their resemblance to the ram-like horns of the Egyptian God Ammon or Amun.

All regular ammonite shells grew out from the centre (the umbilicus) in a spiral in one plane, becoming gradually thicker as it grew. It grew by adding chambers on the shell from the umbilicus, and when it “moved” from an old chamber to a new one (the living chamber), it sealed off the old chamber except for the siphuncle, a “tube” which extended through the shell. The siphuncle could be used to change the air pressure in the sealed-off chambers (known as the phragomocone), so it could rise or descend, rather like a submarine does. The earliest phragomocene chambers tend to get less sediment in them, therefore it is common for crystals to grow in these sections. My ammonite has calcite crystals (which are a very common crystal) in a lot of the phragomocene chambers.

One of the largest ammonites found, Parauzosia seppenradensis

Ammonites would probably have had ten arms (all cephalpods have arms, not legs). It's arms may have surrounded hard aptychi, which may have been the jaw apparatus of the ammonite, similar to the beak in other cephalpods, or the aptychi could have been a sort of head shield, which is seen in living nautiloids. There is very little fossil evidence of the soft parts of ammonites, though it is likely that, in addition to the ten arms, they had an ink sack and a hyonome (see below).

Extant Nautilus shell

Ammonites are fairly distant relatives of the Nautiloids (which include the living Nautilus). The Ancestors of Ammonites branched off from the Nautiloid line (specifically straight-shelled Bactritid nautiloids) before 400 million years ago.

What is the difference between the Nautiloids and the Ammonites, then?

The basic difference is the difference in the sutures. These are the lines where the walls which separate each chamber (called the septa) meet the wall of the shell, which can be seen by looking at the outside of the shell. Nautiloid sutures are generally gentle curves, whereas Ammonite sutures are more wavy. The suture lines are determined by the shape of the septa when it meets the wall of the shell. The difference in the suture lines are caused by the different shapes of septa in Ammonites and Nautiloids. Ammonite septa are more wavy and to a varying extent convex from the front of the shell, whereas Nautiloid septa are smoother and more concave from the front. The shape of the septa can be observed when the ammonite shell is in cross-section. The wavy suture lines enabled ammonites to withstand high pressures whilst having a thinner shell than Nautiloids, though both subclasses were able to living at depth (Nautiloids at a deeper depth than Ammonites).

Ammonites and Nautiloids both have a siphuncle, but the siphuncle of Nautiloids is in the middle of the septa, whereas the ammonite siphuncle is on the outer edge of the shell.