User:GabbLarAnz/sandbox
Topic: Evolution of the body plan for Chondrichthyes; prehistoric to current.
Bibliography
Citations:
Jaw: https://academic.oup.com/biolinnean/article/71/1/165/2638711
Teeth: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0122553
Body plan: https://royalsocietypublishing.org/doi/full/10.1098/rspb.2017.2418
Introduction
There are over 500 species of sharks in the world today, 630 different species of rays and over 200 species of extinct sharks and rays. There were many more fossil records that suggest more than 3,000 types of shark and their relatives existed at one time. Their long history starts in the late Silurian period, about 450 million years ago. It was a time when sea levels were high and coral reefs began to form. The Earth's climate was warm and stable. Molluscs, crinoids and trilobites were some of the only living creatures on the Earth before scorpions and centipedes appeared on the land.
Jaws:
A study of motor activity and the kinematics of head movements during feeding were studied for different species:
- Lemon shark (N. brevirostris): consisted of five sharks and thirty-six capture trials
- Bonnethead (Sphyrna tiburo): consisted of three sharks and fourty-two capture trials
-Spiny dogfish(Squatus acanthias): consisted of eight sharks and fourty-four capture trials
- Atlantic guitar fish (R. Lentiginosus): consisted of five sharks and fourty-two capture trials
These species were studied using electromyography and high speed video during feeding.
A bipolar electrode was inserted into select cranial muscles using gauge hypodermic needles. The sharks were under anesthesia during surgery and after were returned back to the experimental tank to recover.
Once fully recovered, the feeding trails began after the sharks were swimming normally for one hour. Specific fish were given to each species of shark
Results:
When the shark's mouth opens up the proximal end of the hyoid doesn't move posterodorsally like in the teleost. Sharks have a anteroventral rotation that displaces the entire jaw allowing the shark to open its mouth as big as possibly.
The CH muscle originates from the pectoral girdle and inserts into the hyoid arch in almost all chondrichthyes. According to the article:
"Anatomical and experimental evidence on Chondrichthyes supports the role of the CH-coupling in mediating hyoid depression in aquatic lower vertebrates. In Osteichthyes, the CH-coupling also functions to depress the lower jaw but this function has not been found in chondrichthyans"
CH coupling the hyoid suppresses the hyoid arch and expands the buccal cavity. This activity helps in directing the food posteriorly; or being able to move the jaw downwards in order to get food. Also found in sharks the proximal end of the hyoid is connected to the proximal end of the lower jaw by many ligaments. But, this linkage was found to actually not affect the jaw being suppressed or the retraction of the hyoid.
The electromyographic data showed that the activity in the coracohyoideus and the coracoraculis muscles contract after the lower jaw is depressed. The coracohyoideus (CH) coupling does not control the lower jaw but instead the mandibulohyoid (MH) ligaments transmit movements of the hyoid to the mandible. In the lemon shark peak hyoid depression occurs after the CH and the coracoarcualis mucles have stopped there activity. Due to the increase in water pushing against the hyoiod when the mouth is opened, there is a extended duration of the hyoid depression. The peak hyoid occurs during the elevation of the lower jaw.
Teeth
The purpose of this study was to understand the development of the Chondrichthyan teeth. The focus of this study is on batoidea, or rays.
The Batoidea contain over half of all extant species of chondrichthyans, with about 630 species spread between up to 23 families. The Batoidea is now considered to form a sister group to all living sharks, although some studies previously placed them as derived sharks; these forming the Neoselachii (or Elasmobranchii), a clade that excludes many fossil ‘shark’ groups.
Materials/methods
-Dried and prepared dentitions of over 45 batoid species, including examples of all major clades, were studied.
-Additional specimens of adults and late stage embryos of a number of species were selected for more detailed study.
-Late stage embryos (defined as being from within the mother or egg but having at least some developed teeth) of:
Rhinobatos horkelii (Brazilian guitarfish), Myliobatis sp. (eagle rays), Discopyge tschudii (apron ray), Raja clavata (thornback ray), Narcinidae (numbfishes/ electric rays) and Rajidae (s.s.) were studied by x-CT.
Results:
Early ontogeny of rajid and rhino batoids: Upper jaw: alternate positions of the first teeth formed across the width of the jaw.
These teeth were (in the article it says mineralized positive for calcium)
Two parasymphyseal teeth are present next to the jaw symphysis within the upper row of the first tooth
A symphyseal tooth is present in the lower jaw in the first row of teeth
Labial view of the lower dentition of Leucoraja erinacea showing that the first tooth row of 10 positions contains a symphyseal tooth, as in that of Raja clavata
Early ontogeny of Raja clavata: -Segmented developing teeth in embryos
-Upper jaw showing developing teeth as tooth germs under the skin. Numerous papillary projections. fibrocartilaginous fusion of the palaquadrate catilages.
-The upper jaw has a sub-epithelian tooth. Upper jaw also has a lingually rotated jaw showing the symphyseal junction and in the 3rd row of teeth having one fibrocartilaginous fusion
- In both lower jaws there is a symphyseal tooth in the 1st and 3rd row- the tooth positions are can be shifted with growth
-Teeth in second row are developing at later times (arrow heads, start of mineralization) relative to the symphyseal tooth Note in E, cusps have a different orientation in 1st and 2nd rows as tooth germs change their developmental position
Body plan
A study was done investigating the evolutionary body plan of the Gladbachus; which is an extinct species of chondrichthyan. Using the results from the different experiments, first they figured out the length of the head, including the gill skeleton. The processes of computed tomography, anatomical reconstruction, and thin section histology was able to deduce that the head and gills were about twenty-one centimeters; the head and trunk with the caudal region not present is about sixty centimeters. Overall the total length is about eighty centimeters.
The internal skeleton was found to be mainly calcified cartilage lacking a perichondral bone. The Gladbachus also seems to lack a tightly connected tesserae on the cartilage surface that is "the hallmark of confrichthyan skeletal anatomy". The thin selection histology concluded that delineated tesserae are found in another place in the body plan but is concealed under the cartilage surface.
The dermal skeleton has no large plate; the Gladbachus contains large head scales where the lateral line runs between. This shark's scale shape, composition and histology is extremely similar to placoderms and are completely different to the polyodontode scales in Doliodus.
The right side of the postorbital process includes pieces of a jugular canal. There is possible evidence that there was articulation from the jugular canal to the upper jaw. Reconstructions of the vestibular, semicircular canals and ampullary spaces show that the otic capsules were large and widely separated across the midline.
Notably, the scales and teeth bordering the gape are preserved as continuous, subparallel bands spanning the inter-orbital space. The palatoquadrate is generally comparable to that of an early, conventional Chondrichthyan.
Overall, all these puzzle pieces given by Gladbachus and other early chondrichthyans suggest that the morphological differnces in the early members of the chondrichthyan total-group was probably substantially greater.
Putting all this information together:
Sharks and rays in the modern day have very distinct characteristics that weren't present in early generations.
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