Australia: The Land Where Time Began

A biography of the Australian continent 

Cephalaspid biology

Some of the cephalaspid osteostracans have been so well preserved that much of their biology and anatomy have been extracted from specimens of forms such as Hemicyclaspis and Cephalaspis. There are 2 oval openings on the upper surface of the head shield for the eyes, the orbits and in front of them in the midline there is a nasopharyngeal opening, in the form of a narrow slit. Also in the midline, there is a very small pineal opening behind the nasopharyngeal opening, associated with the pineal organ of the brain that is believed may have a light-sensing function.

On the head shield they had 2 dorsal lateral fields and a dorsal field, all 3 being in the form of slight depressions of the head shield lined with small scales. It has been suggested they may have had a sensory function, as they are connected to the auditory region of the brain by large canals in the bone, possibly for carrying nerves or fluid. Suggested functions are movement detection in the water or the detection of electrical fields.

On the posterior margin of the head shield on both sides there were pointed cornua pointing backward and curved notches on either side of the back of the head shield were occupied by pectoral fins.

On the ventral side of the head shield there was a large mouth at the front behind which was a wide area of small ventral scales. And there were 8-10 gill openings on either side of the scale field.

were enclosed by the bone of the head shield. A reconstruction has been carried out of the brain and associated cranial nerves, the major nerves innervating various parts of the head region (Stensiö, 1927), which was made possible by the extensive bony envelope. The large orbits and the inner ear can be clearly seen, as well as the inner ear’s semicircular canals, the balance organ. The brain stem in the midline, composed of the 3 main portions present in living primitive fishes, at the back, leading to the spinal cord, in the middle is the pons, in front is the telencephalon, the forebrain, with an elongated hypophysial sac leading forward from it. By comparing the cranial nerves with those of living vertebrates it has been possible to identify cranial nerve III, associated with eye movement, V2 with the mouth and lip region, VII with the face, IX with the tongue and pharynx, and X with the gill slits and an anterior body. 5 broad canals can be clearly seen connecting the lateral sensory fields to the vestibule in the inner ear.

It has been shown by analysis of their locomotion (Belles-Isles, 1987) that they could swim in a sustained manner, with short bursts of rapid locomotion, as well as carry out fairly delicate movements, in the same manner as extant sharks living on or near the ocean floor. Moving forward would have produced lift, as in side view the head had an aerofoil shape, counteracting the downward thrust of the tail when it moved from side to side, as a result of the tail having an upper lope longer than the lower lobe. The pectoral fins are also to possibly have added some lift to counteract the downward thrust of the tail.

As well as in marine sediments, fossils of cephalaspids have been recovered from deposits of freshwater streams, lakes and deltas. It has been speculated that they may have foraged for detritus on lake bottoms, possibly moving the body over the sediment by movements of their muscular pectoral fins. It is also believed they could swim great distance, such as searching for new feeding grounds or in short bursts of rapid swimming to avoid predators.

Sources & Further reading

  1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.

 

 

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Last updated 21/10/2016

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading