Thougts on Palliocerida

Is Palliocerida a valid  order within the Nautiloidea

Marek in 1998, established the Palliocerida as a separate order of cephalopods within the Nautiloidea for orthoconic and mildly cyrtoconic forms with incomplete connecting rings in the siphuncle  such that the space within the camerae and siphuncle interior are contiguous,  and in which the camerae, or chambers, contain organic calcite deposits. 

The primary question is, do incomplete connecting rings justify the creation of  a new order.  In my view no.    

First of all,  open or incomplete connecting rings,  that is with a gap or gaps open to the chambers, would greatly impair normal siphuncle function, which is to dewater the final chamber of the phragmocone after it's formed and to adjust buoyancy throughout during life for mobility.   Open or missing connecting  are more likely the result of decalification resulting in none preservation or some  post mortem diagenetic process.

Second of all, open or incomplete connecting rings or not necessary in order to have cameral deposits Genera in found in different families and orders, such as Actinoceras and related genera in the  Actinocerida;  Michelinoceras in the Orthocerida;  Pseudocyrtoceras in the Pseudorthocerida; Westonoceras in the  Discosorida as well as  Campbelloceras, Curtoceras,and Lituites, in  the  Tarphycerida, to name but a few, all have cameral deposits and except for some Lituites, well developed siphuncles will complete connecting rings.  




Third,  I see no  phylogenetic reason for the order.  Therefor considering the arguments given,  it seems best that the forms included in the Palliocerida Marek 1998 ,  such as Plagiostomoceras be returned to the Orthocerida and the Palliocerida be abandoned 


 

Endocerid vs Actinocerid siphuncle

The  Endocerida  and Actinocerida were important contributors to early Paleozoic, mainly Ordovician, marine faunas.  Both cephalopod orders are well represented by straight (orthoconic) genera, some which produced large shells, some in excess of 3 or 4 meters. What distinguished them, above all else, is their siphuncle.  The picture at the right 
shows a segment of an endocerid siphuncle on top, with nearly straight edge and sectioned endocones and of an actinocerid below showing expanded segments along the margin and parietal deposits within.

The picture is a composit of two specimens housed at the New Mexico Museum of Natural History and Science in Albuqerque, New Mexico.                                                     

                                                                          

Large Newfoundland Tarphys

 
The New Mexico Museum of Natural History and Science in Albuquerque NM has in its fossil collection a number of large tarphycerids, an order containing the earliest coiled nautiloid cephalopods, from Newfoundland, Canada, originally belonging to the late Dr Rousseau Flower.   Two,  Loganites and Bennettoceras, were named by him but descriptions are lacking.  Others presumed also to be from Newfoundlland are unidentified.  
                        
                                                                                
Loganites, upper left,  Bennettoceras, middle left, and a large unnamed tarphycerid, bottom right.  All are external views.  Much of the matrix was artificially stripped away using Photoshop. Loganites is on the order of 7 in in diameter ( about 18 cm),  Bennettoceras is somewhat smaller, about 5 in ( 12.5 cm).   The straight portion of the unnamed specimen is about 7 in (18cm) as well.   All are internal molds.  All are Ordovician in age

A barrandeocerid section

Shown here is a section of a barrandeocid,, housed in the collections of the New Mexico Museum of Natural History and Science,  Albuquerque, NM, http://nmnautralhistory.org.  The specimen, P-56465, is about 8 cm long (about 3 in.).

The interpretation is mine, based  on the close spaced septa and narrow tubular siphuncle.  I have yet to assign a genus name. Location is unknown, but based in the overall color and general texture I'd say it came from the Ordovician of northeastern North America, most likely Newfoundland. 

The first picture, on the left, shows the overall specimen , what's left of it, an arc segment of one of the whorls. The second , below,, shows the specimen in greater detail with the septa and narrow, tubular siphuncle, located slightly below the center, clearly shown. 

                                                                          

Ammonites vs Nautiloids

Ammonites, subclass Ammonoidea, and nautilids, subclass Nautiloidea are first of all both cephalopods with external, chambered, shells. So how are they different and how are they alike, other than for their external shells

Nautiloids came first, having first appeared in the Late Cambrian, and gave rise to the ammonoids in the Early Devonian.  Some sticklers for cladistic purity might say that ammoinoids are simply a type of nautilids.   From a purely evolutionary perspective this could be said to be so.  However ammonoids are sufficient distinct to warrant being put in a separate equal taxon to the Nautilidea.  Why?  

Nautiloides have concave septa dividing the chambers of the phragmocone, as viewed from to front, looking back toward the apex of the shell.  Sutures, marking the junction of the septa with be body of the shell are generally straight and transverse or may have shallow saddles and lobes.  A few later forms from the Late Cretaceous and early Cenozoic even have goniatitic sutures, resembling those of the Late Paleozoic ammonoid Goniatitida. None have sutures nearly as complex as those found in Mesozoic ammonoids 


The siphuncle, the tube that interconnects the chambers in cephalopod shells is varied and often complex in  nautiloids but is always a simple narrow tube in ammonoids. Ammonoid siphuncles are commonly although not exclusively found along are ventral or outer margin. Nautiloid siphuncles are commonly ventral or central or somewhere in between. In both groups there are forms with dorsal siphuncles found along the near the dorsal or inner margin


In summary, one can study ammonoids from the outside, looking at general form, ornament, and sutures.  One has to study nautiloides from the inside to get a true picture, looking at the internal details of the shell, especially those of the siphuncle.

Some early actinocerids

Here are holotypes of three actinocerids (order Actinocerida) housed at the New Mexico Museum of Natural History and Science in Albquerque, N.M. They all belong to the Wutinoceratidae, which is an early actinocerid family and all three come from the Pallisaria zone in the Antelope Valley Limestone near Beatty, Nevada. They are, with museum catalogue numbers:

Adamsoceras leonardi P-42779
Cyrtonybyoceras adamsi P-42976
Wutinoceras huygenae P- 42877




All three are from the Whiterock stage at the beginning of the Middle Ordovician. Wutinoceras is considered the ancestral form, which gave rise to the other two, as well as to the Armenoceratidae and Actinoceratidae. Adamsoceras is thought to have given rise to the Ormoceratiae. Cyrtonybyoceras apparently left no descendants.

What about Barrandeocerida

Flower, in Flower and Kummel, in A Classiification of the Nautiloidea, Journal of Paleontology Sept 1950, estalished the Barrandeocerida as a separate order, derived from the Tarphycerida. Barrandeocerids first appear in the Middle Ordovician, later than the Tarphycerida which first appear in the Lower Ordovician. Moreover Barrandeocerids have derived (evolved) characters, the principal one being thin connecting rings. So it makes, or made, sense to separate the two on an equal basis.

As recently as 1976, Flower in his paper of Ordovician Cephalopod Faunas, published by the Palaeontological Society ( Gr Br) separated the Barradeocerida from the Tarphycerida, deriving the former from the latter. This changed in 1984.

Flower, in Bodeiceras; a New Mohawkina Oxycone; ... Journal of Paleontology Nov. 1984, showed that the first two families of the Barrandeocerida, the Barradeoceratidae and Plectoceratidae are derived from different genera in the Tarphyceratidae, making the Barrandeocerida polyphyletic and therefore invalid, which according to Flower, should be abolished. Families that were included in the Barrandeocerida now belong in the Tarphycerida, which as a result has become greatly expanded. However Teichert, 1988, in has review paper on Main Features of Cephalopod Evolution in The Mollusca Vol 12, Academic Press, retained the Barrandeocerida as a distiinct group, but as a suborder of the Tarphycerida, the Barrandeocerina. The other suborder of course being the Tarphycerina.

Flower's claim that the Barrandeoceratidae are derived from Centrotarphyceras and that the Plectoceratidae are derived from Campbelloceras is not stated with clear evidence, but is left for the reader to simply accept, or reject. For this reason the claim that the Barrandeocerida (sensu Flower 1950) is polyphyletic can not be taken as an absolute. It is obvious however that barrandeocerids form a group that evolved from earlier tarphycerids and might just as well be included as a suborder pending further evidence of separate origins for the Barradeoceratidae and Plectoceratidae.