[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Cool Jurassic Marine fauna



> Â Are there any late surviving placodonts that made it
> into the the Jurassic?


No.  Unfortunately, because I'd love to see a placodont come to life with CGI.  


One recent paper restores placodonts as Triassic analogs of modern dugongs and 
seacows...



Diedrich, C.G. (2010).  Palaeoecology of Placodus gigas (Reptilia) and other 
placodontids â Middle Triassic macroalgae feeders in the Germanic Basin of 
central Europe â and evidence for convergent evolution with Sirenia.  
Palaeogeography, Palaeoclimatology, Palaeoecology 285: 287â306.

Abstract: "The classical descriptions of Middle Triassic marine Placodus gigas 
Agassiz, 1833 (Reptilia) from the Germanic Basin of central Europe as being 
shell-crushing durophagous placodontids are revised in this paper through 
analyses in convergent anatomy. In particular, the jaw morphologies of three 
placodontid genera, Paraplacodus, Placodus and Cyamodus, are compared to those 
of dugongs (Mammalia) such as the central European Tertiary Halitherium 
schinzii and the modern Dugong dugon of the Arabian Gulf. The anatomies of 
Paraplacodus, Placodus and Cyamodus exhibit convergences to Halitherium and 
Dugong. Whereas mammalian dugongs developed pachyostotic thoracic ribs to 
enhance their body weight, the placodontid reptiles achieved a similar result 
in different ways: Paraplacodus developed enlarged thoracic ribs; Placodus had 
pachyostotic gastral ribs, and Cyamodus had a thoracic osteoderm shield. The 
teeth of the placodontids are also convergent with
 those of Halitherium and Dugong in their general function and jaw morphology. 
Whereas Halitherium and the modern Dugong possess a horny oral pad and 
counterpart, and a specialized rasp-like tongue with which to grind the 
seagrass and its roots, placodontids had large teeth that covered thewhole of 
their upper and lower jaws forming a similar crushing or grinding pad. Both of 
the extinct groups must have fed on sea-plants, as does the modern Dugong, 
although Halitherium possibly fed on both seagrass and macroalgae. A study of 
the wear stages of many Placodus teeth, skulls and jaws has revealed a large 
proportion of highly worn anterior teeth, indicating a usage similar to that of 
the procumbent front teeth of modern Dugong which are used to scrape plant 
roots fromthe seafloor. In contrast, highly worn (wear stage 3) teeth are rare 
(0.5%) amongst all other upper palatal, maxillary, or lower-jaw dentary teeth, 
suggesting a relatively soft diet. Placodus
 must have used their broadly spaced anterior teeth, to dig macroalgae from 
carbonate sands in shallow marine, sand bar environments; indirect evidence for 
the existence of such environments is provided by the benthic communities of 
the Germanic Basin and the northern Tethys. Sea-plants would have been only 
crushed and swallowed by the placodontids and not chewed with jaw rotation, in 
a similar feeding strategy to that used by modern Dugong feeding on seagrass."


Cheers

Tim