Monday, May 25, 2009

The Devonian Fishes that Got Away

In addition to remembrance ceremonies paying tribute to the men and women of the armed services, Memorial Day has become a day in which Americans head to the great out-of-doors to enjoy friends, family, picnics and nature. With this tradition in mind, I thought it fitting to briefly mention a couple of fish stories. These not so tall-tales are about a couple of good catches from the past year that somehow managed to get away from me.

The first story received quite a bit of publicity, it was in the news about a year ago and may be old news for some - but it's a fish worth a second look!

The fossil Materpiscis attenboroughi is a 380-million year old placoderm that was found with an intact umbilical cord! According to Dr. John Long of the Museum Victoria,“The discovery is certainly one of the most extraordinary fossil finds ever made. It is not only the first time ever that a fossil embryo has been found with an umbilical cord, but it is also the oldest known example of any creature giving birth to live young. The existence of the embryo and umbilical cord within the specimen provides scientists with the first ever example of internal fertilisation - i.e sex - confirming that some placoderms had remarkably advanced reproductive biology. This discovery changes our understanding of the evolution of vertebrates.”

Here's a Youtube Video (originally from Nature):

Long, J., Trinajstic, K., & Johanson, Z. (2009). Devonian arthrodire embryos and the origin of internal fertilization in vertebrates Nature, 457 (7233), 1124-1127 DOI: 10.1038/nature07732

Abstract: Evidence of reproductive biology is extremely rare in the fossil record. Recently the first known embryos were discovered within the Placodermi, an extinct class of armoured fish, indicating a viviparous mode of reproduction in a vertebrate group outside the crown-group Gnathostomata (Chondrichthyes and Osteichthyes). These embryos were found in ptyctodontids, a small group of placoderms phylogenetically basal to the largest group, the Arthrodira. Here we report the discovery of embryos in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia (approximately 380 million years ago), providing the first evidence, to our knowledge, for reproduction using internal fertilization in this diverse group. We show that Incisoscutum and some phyllolepid arthrodires possessed pelvic girdles with long basipterygia that articulated distally with an additional cartilaginous element or series, as in chondrichthyans, indicating that the pelvic fin was used in copulation. As homology between similar pelvic girdle skeletal structures in ptyctodontids, arthrodires and chondrichthyans is difficult to reconcile in the light of current phylogenies of lower gnathostomes, we explain these similarities as being most likely due to convergence (homoplasy). These new finds confirm that reproduction by internal fertilization and viviparity was much more widespread in the earliest gnathostomes than had been previously appreciated.

Check out the Museum Victoria's Website for more info on Mother Materpiscis.

The second Devonian fish story that slipped away is one that has been retold since 1892 and really has some teeth to it!

In fact, teeth are all that it has to it – fossil teeth. Back in 1892 Woodward described the Devonian chondrichthyan Protodus jexi from the Lower Devonian Campbellton Formation in New Brunswick based on teeth recovered from the site. Several months ago, Susan Turner and Randall Miller published a follow-up examination of these teeth in Acta Geologica Polonica and determined the fish to be representative of the first predatory fish.

Here’s their conclusion:
Protodus jexi is known from one locality on the Campbellton shoreline of northern New Brunswick and represents the first chondrichthyan with predator style teeth. Given the nature of shark dentitions, the known 60 or so teeth might belong to one dentition. The paratypes and probably the syntypes include a tooth file of four associated dagger-like teeth with serrated cristae. The large subrectangular to D-shaped Protodus toothbase is similar and might be phylogenetically related to the cladodont type. Contemporaries Celtiberina and Stigmodus might be closely related or the similar tooth features might reflect functionality, with large laterally extended bases providing such ‘protodontidid’ teeth with a ‘solid’ root resisting movement and assisting an efficient predatory bite.

A Couple of Fossil Protodus jexi Teeth. Source Turner & Miller Article

SUSAN TURNER,RANDALL F. MILLER (2008). Protodus jexiWoodward, 1892 (Chondrichthyes),from the Lower Devonian Campbellton Formation, New Brunswick, Canada Acta Geologica Polonica, 58 (2), 133-145

The article is available HERE.

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