Belemnites

The cigar-shaped fossils of Belemnites are common in the West Coast’s Jurassic and Cretaceous. The solid portion of the shell commonly found represents a portion of the guard, devoid of the phragmocone and pro-ostracum. The most massive element of a Belemnoid’s shell is the guard, or rostrum. Unrepresented in the shell of any other cephalopod, the rostrum is fossilized as calcite leading to durability and frequent preservation. The phragmocone is the chambered portion of the shell and, as in nautiloids and ammonites, there is a siphuncle. However; the belemnite siphuncle is straight, without septal necks. Each septal chamber of a baculite, ammonite, or nautiloid has shell encasing the siphuncle which is pinched as a collar or neck at septal junctions. The pro-ostracum is a projection of the phragmocone’s dorsal surface extending over the visceral body much like the pen in today’s squid. Belemnoids have been found from the Mississipian age, but true belemnites didn’t evolve until the Triassic, and they persisted through to the Eocene, although they are very rare after the Cretaceous. Exceptionally well preserved belemnites from Europe have shown double rows of chitinoid hooks called onychites. Onychites have been found in rows of eight and ten, generally interpreted as arms with no long grasping tentacles. The internal structures of all belemnoids are the same, but the animals’ proportions varied greatly. In the Triassic, Ausseites had a small guard with a long phragmocone, while others like the Jurassic’s Belemnoteuthis had a very thin guard with a short fat phragmocone. Guards can be short or very long, or even diverting from the conical form. In the Cretaceous
Hibolites, the guard is hastate, getting thin just before the phragmocone, while Duvalia has a guard that flattens out like a vertical fin. Taxonomy in belemnites is based on the shape of the rostrum, size, cross-section, and grooves. Seen in cross-section the rostrum has rings that can be interpreted much like those of a tree; the average belemnoid was thought to live about four or five years. Based on the rostrum, the average belemnite animal was 300 to 500mm and no more than half a metre in length. Of course, there are also extremes such as Neohibolites which was about 100 mm long, depending on its arm length; and a rostrum from Indonesia is so big it suggests a belemnite that may have been 5 meters long! Rarely found compared to the guard, the protoconch of the average belemnite is a small ball at the phragmocone’s tip. Belemnoid protoconchs from Harrison Lake are 3/4 of a millimeter across, and the phragmocone has 40 septa. For comparison, a new-born ammonite from the same locality is a millimeter in size with a protoconch of about the same size as the belemnite. The small sized new-born belemnite would be reliant on the planktonic world just as the ammonite, and the reduction of a large percentage of the plankton at the end of the Cretaceous could have been responsible for at least part of the
belemnites’ demise.The cigar-shaped fossils of Belemnites are common in the West Coast’s Jurassic and Cretaceous. The solid portion of the shell commonly found represents a portion of the guard, devoid of the phragmocone and pro-ostracum.
The most massive element of a Belemnoid’s shell is the guard, or rostrum. Unrepresented in the shell of any other cephalopod, the rostrum is fossilized as calcite leading to durability and frequent preservation. The phragmocone is the chambered portion of the shell and, as in nautiloids and ammonites, there is a siphuncle. However; the belemnite siphuncle is straight, without septal necks. Each septal chamber of a baculite, ammonite, or nautiloid has shell encasing the siphuncle which is pinched as a collar or neck at septal junctions. The pro-ostracum is a projection of the phragmocone’s dorsal surface extending over the visceral body much like the pen in today’s squid.Belemnoids have been found from the Mississipian age, but true belemnites didn’t evolve until the Triassic, and they persisted through to the Eocene, although they are very rare after the Cretaceous. Exceptionally well preserved belemnites from Europe have shown double rows of chitinoid hooks called onychites. Onychites have been found in rows of eight and ten, generally interpreted as arms with no long grasping tentacles. The internal structures of all belemnoids are the same, but the animals’ proportions varied greatly. In the Triassic, Ausseites had a small guard with a long phragmocone, while others like the Jurassic’s Belemnoteuthis had a very thin guard with a short fat phragmocone. Guards can be short or very long, or even diverting from the conical form.

Figure 2. Cutaway of a belemnite showing the rostrum, phragmocone, and pro-ostracum in life position.

 In the Cretaceous Hibolites, the guard is hastate, getting thin just before the phragmocone, while Duvalia has a guard that flattens out like a vertical fin. Taxonomy in belemnites is based on the shape of the rostrum, size, cross-section, and grooves. Seen in cross-section the rostrum has rings that can be interpreted much like those of a tree; the average belemnoid was thought to live about four or five years. Based on the rostrum, the average belemnite animal was 300 to 500mm and no more than half a metre in length. Of course, there are also extremes such as Neohibolites which was about 100 mm long, depending on its arm length; and a rostrum from Indonesia is so big it suggests a belemnite that may have been 5 meters long! Rarely found compared to the guard, the protoconch of the average belemnite is a small ball at the phragmocone’s tip. Belemnoid protoconchs from Harrison Lake are 3/4 of a millimeter across, and the phragmocone has 40 septa. For comparison, a new-born ammonite from the same locality is a millimeter in size with a protoconch of about the same size as the belemnite. The small sized new-born belemnite would be reliant on the planktonic world just as the ammonite, and the reduction of a large percentage of the plankton at the end of the Cretaceous could have been responsible for at least part of the
belemnites’ demise.

 

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