Saturday 24 December 2016

Ontogeny in Siphonodellid Conodonts.

Conodonts are curious microfossils found in rocks from the Cambrian to the Trissic in many parts of the world. They are tooth-like structures, comprising a number of distinct elements made from the same material as the teeth of Vertebrates, but distinct from the teeth of any known Vertebrate, with each Conodont apparently havins a unique set of these structures. For many years the animals that produced these fossils were a mystery, but they are now known to have been Eel-like organisms, considered to be Chordates closely related to, but outside the Vertebrate group. Conodonts are often extremely numerous, and due to their high species turnover are often used to date rocks. However, like other aninals, Conodonts changed shape as they grew, a process known as ontogeny, and which means thay for accurate use as a dating tool biostratigraphers (palaeontologists that use fossils to date rocks) must be confident that they are able to identify all growth stages in an animal, so that they do not mistake assemblages of juvenile animals (which may be a seasonal phenomenon) as rocks of a different age.

In a paper published in the journal Vestnik in August 2016, Artem Plotitsyn and Andrey Zhuravlev of the Institute of Geology Komi present ontogentic series for the Pa elements (tooth-like structures from the rear of the apparatus) of Conodonts of the genus Siphonodella, which is found in Late Devonian to Early Carboniferous rocks in many locations, species of which are considered very important for dating rocks of the Early Carboniferous.

Plotitsyn and Zhuravlev examined Conodont-elements from the Polar and Subpolar Urals, Chernyshev and Pechora ridges, with much of the material coming from the Kozhym River and Konstantinov Creek sections, the Vangyr River section, the Iz’yayol Creek section, the Malaya Usa River section and the Kamenka River section. This collection yielded over 3000 individual specimens from the species Siphonodella duplicata, Siphonodella quasinuda, Siphonodella semichatovae, Siphonodella lobata, Siphonodella crenulata, Siphonodella hassi, Siphonodella obsoleta, Siphonodella sandbergi, Siphonodella belkai, Siphonodella quadruplicata, Siphonodella cooperi and Siphonodella isosticha.

Morphological terminology of Pa-element of Siphonodella. (A) Measurements: (L) element length; (W) platform width; (Lfb) length of the free blade; (Lbc) length of the anterior process; (Lpp) length of the posterior process; (B) lamella sets in the platform of the juvenile Pa-element of Siphonodella quadruplicata, transmitted light image. Plotitsyn & Zhuravlev (2016).

The Conodonts were divided into three growth stages, the first two of which were considered to be juveniles. The fist stage has up to three lamellae sets on the platform and lacks rostral ridges and any form of platform ornamentation, the second stage has 3-5 lamelae sets and have rostral ridges and ornamentation on the outer part of the platfrom, while the third stage has more than five lamellae sets and shows full ornamentation.

Based upon these growth stages Plotitsyn and Zhuravlev were able to divide the Conodonts into two distinct developmental groups.

The first group comprised Conodonts in which the Pa-element bore up to two rostral ridges and did not undergo any significant structural modification during development. This group comprised the species Siphonodella duplicata, Siphonodella quasinuda, Siphonodella semichatovae and Siphonodella lobata.

 Ontogenetic successions of the characteristic Siphonodella species possessing up to two rostral ridges. Plotitsyn & Zhuravlev (2016).

 The second group comprised Conodonts in which the Pa-element had more than two rostral ridges at full development. The earliest stages of the members of this group were often similar enough to be hard or impossible to distinguish, but they underwent significant modification during development, with later stages being very distinct. This group comprised the species Siphonodella belkai, Siphonodella obsoleta, Siphonodella cooperi and Siphonodella quadruplicata.

 Ontogenetic successions of the characteristic Siphonodella species possessing more than two rostral ridges at late stage of ontogeny. The vertical grey stripes mark ontogenetic stages, at which species can’t be confidently separated. Plotitsyn & Zhuravlev (2016).

See also...
Follow Sciency Thoughts on Facebook.