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.