No more Basslerocerida

The late Rousseau Flower in Flower and Kummel, 1950, in the Journal of Paleontology, Sept 1950 set up the Bassleroceratida as an order of nautiloid cephalopods intermediary between the more primitive, generally straight shelled Ellesmerocerida and the more evolved, coiled Tarphycerida and later Barrandeocerida. Basslerocerids are exemplified by the upwardly curved genus Bassleroceras in which the lower or ventral side is longitudinally convex and the upper or dosal side is curved in the opposite sense. The order at that time included two families, the Bassleroceratidae and Graciloceratidae.

Seven years later, Flower 1957 in Flower and Teichert, in a University of Kansas publication on the Discosorida, abandoned the Bassleroceratida and instead included the Bassleroceratidae, with thick connecting rings in the Tarphycerida and the Graciloceratidae with thin connecting rings in the Oncocerida.

Taking a somewhat difference approach, W.M, Furnish and Brian Glenister in the original, 1964 edition of Part K of the Treatise on Invertebrate Paleontology, included the Bassleroceratidae in the ancestral Ellesmerocerida . Walter Sweet on the otherhand followed Flower's perspective and retained the Graciloceratidae in the Oncocerida. Later, Flower, 1976, in a publication of the Palaeontological Society ( Gr Br) on Ordovician Cephalopod Faunas, shows the Bassleroceratidae in the Tarphyceratida (same as Tarphycerida), with the earlier more primitive forms giving rise to the Tarphyceratidae and the more derived forms later giving rise to the Oncocerida following his original (1957) idea.

So, to conclude, no more Basslerocerida. The order should be regarded as unnecessary, more so than "invalid". Never-the-less Shevyrev, 2006, according to the Paleobiology Database website, retained the Basslerocerida as a distinct nautiloid order, with the Barrandeocerina its sole suborder. This follows neither the sense of Flower (1950, 1957, and 1976), of the Treatise Part K, 1964, Teichert et al, or of Teichert 1988. For one thing the Barrandeocerina ( ex Barrandeocerida) are clearly derived from the Tarphyceratidae and therefore cannot be part of the Basslerocerida, regardless. Since what is presented on line is not necessarily what is in the original, perhaps Basslerocerida should really be Barandeoceida, which makes more sense.

Heteromorphs

Heteromorphic ammonites, or simply heteromorphs, hold a great fascination for some. It's easy to see why, what with their sometimes odd shapes that look as though the animal wasn't quite sure what it was doing. Shapes vary from simple coils that straighten out, some ending in hooks, to helically wound coils that then become planar, to straight shells that shift to the opposite direction connected by U-turns; and more and in between.

The term, heteromorph, or heteromorphic, means having more than one form or shape. While many heteromortph ammonites are heteromorphic, not all truly are. Some like the Crioceratidae have what are really openly coiled monomorphic (single form) shells, and others like the Turrilitidae have tall spired helically wound shells that are also monomorphic. Because these are related to truly heteromorphic forms, they are generally included in the heteromorphs. Truly hetermorphic forms include the Ancyloceratidae, Macroscaphitidae, Heteroceratidae, Ptychoceratidae, and even the Baculitidae.

A better. and more scientific approach is to refer to these taxonomically, to the Ancyloceratina for Cretaceous forms, or to the Ancyhloceratina and Turrilitina if the distinction is correst, and to the Choristcerataceae (Choristoceratoidea if you want to go along with the rank ambiguous requirement of the ICZN) for Upper Triassic forms.

Actinocerids vs Endocerids

Actincocerids (Order Actinocerida) and endocerids (Order Endocerida) are generally straight shelled nautiloid cephalopods that lived during the early and middle Paleozoic; Middle Ordovician to Early Pennsylvanian for the actinocerids, Early Ordovician to Silurian for the endocerids. Externally they were mostly similar although one branch within the Endocerida has downwardly curved, endogastric shells, reverse of the progenetors of coiled formed, and some within the Actinocerida became broad and rather flattened. Both, especially for the straight shelled forms, lived with a horizontal orientation as detarmed by the common generlly ventral siphuncles found in both and the apical counterbalancing internal deposits. This is where the similarities ended.

Internally actinocerids and endocerids are rather different. This is especially true of their siphuncles. Actinoceratids were heavy, with cameral deposits and expanded siphuncle segments containing deposits pierced by a complex canal system. Endoceratids on the other hand were light, the only significant mass being in the endocones found in the more after part of an often wide siphuncle. In the endocerids the viscera (body organs) probably occupied the forward, empty part of the siphuncle as well as the body chamber at the front of the shell while in the actinocerids the organs were restricted to the body chamber with the siphuncle containing only nerves, blood vessels and related tissue. With endocerids in general the siphuncle with endocones is often the only fossil that remains. On the other hand it is common to fine entire actinocerid phragmocones or segments there of. Obviously these were very different animals with different life styles .

Lifestyle of the Trocholitidae

Trocholitids are coiled nautiloid cephalopods belonging to the Tarphycerida, from the latter part of the Ordovician Period. are distinguished by the presence of a dorsal siphuncle, and as often the case, some what anisotrophic coiling such that one side is more concave, or less convex, than the other. In the jargon of cephalopod morphology this asymmetry is known as torticonic, as opposed to trochoidal as used to describe the anisotrophic coiling in gastropods, resulting a a spire. As to why these nautiloids aren't called the Torticonidae, your guess is as good as mine. So much for the vagaries of nomenclature..

In straight or moderately curved forms, dorsal siphuncles are located anywhere from slightly above the center to along the upper, or dorsal margin. In vertically oriented coiled forms, where the whorls at the top are upside down, dorsal siphuncles run through the inner part of the coils to along the inner margin. In vertically oriented forms they indicate that the last formed septa are high in the shell where the siphuncle is actually low and most efficiently placed to remove the seawater from the last formed chamber.

Vertically oriented trocholitids, more concave, or less convex, on the right side would tend to veer to the right going ahead or to the rear, like a ship with the rudder shifted right. These with the left side more concave, or less convex, would likewise veer to the left, like a ship with the rudder shifted left. This seems to be more of a disadvantage to coiled forms capable of swimming off the sea floor than an advantage. So what was its purpose other than perhaps to confused predator or prey. Perhaps the answer lies in a different orientation during life.

Could it be that at least some trocholitids lived out most of their lives lying on the sea floor on one side, like flounders. The dorsal siphuncle doesn't seem to be any particular advantage, or disadvantage here, but even a slightly trochoidal coiling does. One side, probably the upper side, being more convex or domed than the other might have helped bottom currents hold the shell to the sea floor. In this scenario these trocholitids would have spent their time relaxed on the sea floor, ready to ambush the unexpected passer by, only to used there tentacles to help shift position on occasion.

A siphunclde found

Shown here is part of the siphuncle of the Late Paleozoic nautiloid cephalopod Domatoceras that was found in a specimen at the New Mexico Museum of Natural History and Science. This may be a one-of-a-kind find that adds to our understanding of this genus.


Descriptions of Domatoceras so far have been limited to the evolutely coiled shell with a subquadrate whorl section.
The siphuncle is only mentioned in passing as being subcentral and probably orthochoanitic. Subcentral it is but orthochoanitic, at least in this case, it is not.



The picture to the left shows the specimen in which the siphuncle was found, exposed in the second whorl near the bottom.

Are endocerids all that different.

Some cephalopod classifications have the Order Endocerida separated from the Nautiloidea and put instead in the Subclass Endoceroidea (or Endoceratoidea), following the scheme of Curt Teichert in the Treatise on Invertebrate Paleontology. The question is, are they all that different so as to warrant their exclusion from the Nautiloidea and inclusion in another subclass. Teichert thought so, as he did for the Actinocerida, placing them in the Actinoceratoidea (or Actinoceroidea). Russeau Flower thought not, on both accounts, for which I'm in full agreement.

First of all, in brief, what are endocerids. They are cephalopods with straight or downwardly curved shells, close spaced septa, chaambers free of organic deposits, and large ventral siphuncles containing conical deposits concentrated toward the rear known as endocones. The septa are dish-shaped, concave from the front. Septal necks, protrusions of the septa that make up part of the siphuncle point to the rear. Both are characters of the Nautiloidea. These don't necessarily limit the Endocerida to the Nautiloideaas their could be other things calling for their separation. So let's take a quick look at Teichert's reasoning.

The addition of the Subclass Endoceroidea is based on the comination of the Endocerida with the Intejocerida, which turms out to be a group of unrelated cephalopods (according to Flower) whose common charater happens to be the presence of radial lamenllae in the siphuncle. In other characters such as siphuncle location they are quite different. I'll try and touch on the Intejocerida later on. Meanwhile there doesn't seem to be any reason to exclude the Endocerida from the Nautiloidea.

They aren't all that different

What happened to the Basslerocerida.

The order Basslerocerida was established (erected in the odd parlance) by Rousseau Flower (1950) for nautiloides intermediate between straight shelled Ellesmerocerida and coiled Tarphycerida. Species included are in general, exgastrically cyrotoconic, curved like an old rocking chair rocker. Through the powers of evolution the upwardly curved shells became wound back on them selves, first as open, gyroconic spirals, then serpenticones with whorls in contact, true tarphycerids So what happened to them other than that.

Well first of all they did not disappear from the fossil record or vanish from time. The order simply stopped being used, except in historical reference. Flower (1976) simply abandoned the order and included the Bassleroceratidae in the Tarphycerida as the ancestral family, whereas William Furnish and Brian Glennister previously (1964) had included them in the Ellesmerocerida. Take your pick. Either works. As for me, I go along this time with Furnish and Glennister, for what ever it's worth which probably isn't all that much, leaving Tarphycerida for truly coiled forms.

Rousseau Flower and Bernhard Kummel, 1950. A Classification of the Nautiloidea - Journal of Paleontology
Rousseau Flower, 1976, Ordovician Cephalopod Faunas and Their Role in Correlation -Palaeontological Assoc.
William Furnish and Brian Glennister, 1964. Nautiloidea - Ellesmerocerida. --Treatise on Invertebrate Paleontology