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means that the two modes of octopod breeding occurred in ammonoids and controlled their
distribution. The similarity of the argonautid brood cases to Upper Cretaceous ammonites
strengthens these ammonoid-octopodid relationships, suggesting phylogenetic connections.
Lewy (1996) suggested that octopods descended from ammonoids in which the conch
degenerated until total loss like in opistobranch gastropods. This evolutionary trend can be
explained by the diversification of fish, sharks, marine reptiles and belemnites (Lewy, 2009)
predating, among others on slow swimming ammonites which their conch did not protect
anymore from these skillful hunters. On the other hand, the conch limited the expansion of
the mantle cavity and hence the expelled water jet which controlled swimming speed. The
lack of an external conch overcame these restrictions and improved maneuverability, while
additional strategies improved octopods' escape from predators. The earliest octopods
descended from several ammonoid groups, whereby some carried out the 'stationary' mode
of breeding and the others- the 'pelagic' mode. These conchless creatures were
physiologically required to carry out the two modes of breeding in which the 'pelagic' one a
floating egg-case was needed. Empty ammonites floated for some time over the Jurassic and
Cretaceous seas before sinking into the depths. These were occupied by the relevant
octopods and amended into suitable egg-cases. In Late Cretaceous times, the common
ammonites
Hoplitoplacenticeras
,
Jeletkytes
and
Phylloceras
were amended into floating egg-
cases by the breaking off of the terminal part of the conch and its extention in an uncoiling
shape, enabling the octopod female to enter and care for the brood. This added part was
probably made of conchiolin secreted from glands developed at the end of two tentacles as
reflected by extant argonautid octopods. The disappearance of floating conchs in earliest
Cenozoic times forced the surviving octopods to produce the whole brood-case, which they
did in the shape of the Late Cretaceous ammonites, which their ancestors had learned to
construct. The short longevity of extant octopods (1-3 years), when applied to ammonoids
provided an explanation to the function of the fluted margins of ammonite septa (Lewy,
2002a), ammonoid high evolution rate and other phenomena in ammonoids (Lewy, 2002b),
corroborating the deduced ammonoid-octopodid genetic relationships as hinted by other
common characteristics (Lewy, 1996). Accordingly, the order Ammonoidea did not
completely disappear at the K-T boundary, but only the conch-bearing ones. In this respect
the Ammonoidea are comparable to those dinosaurs from which the birds descended.
The endoskeleton of the cephalopod order of
Belemnitida
has been suggested to balance the
horizontal orientation in the water while the belemnoid preys and swallows skeletal
fragments (Lewy, 2009). These were mainly made of calcareous composition of a specific
gravity twice that of flesh, being temporarily stored in the frontal crop. The change in
weight in the anterior side through the accumulation and regurgitation of these fragments
was balanced by water-gas exchange in the phragmocone. The rapid evolution of the
belemnites since the Early Jurassic was associated with the appearance of calcitic opercula
(aptychi) in ammonites. This calcification of the pair of 'wings' of the lower jaw in the shape
of the aperture was intended to protect the ammonoid by preventing crustacean claws,
belemnite tentacles and other means of predation from penetrating into the conch. Most
aptychi are found associated with belemnites suggesting prey-predator relationships. The
same ammonite genera (e.g.,
Baculites
) in regions without belemnites, lack any associated
aptychi plates (Lewy, 2009). Some Late Cretaceous belemnites reduced the size of the guard
(rostrum) up to complete disappearance (e.g.,
Naefia
,
Groenlandibelus
) suggesting a change in
their diet comprising less skeletal parts- a fact which was attributed to the profound
reduction in the abundance of ammonites as prey. These latest Cretaceous belemnites share
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