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Perhaps we are going about this all wrong. Instead of pointing to
analyses, let us test the suggestion that *Mesosaurus* is similar to
*Lariosaurus,* a nothosaur sauropterygian (and a basal lepidosauromorphan
-- sister group to plesiosaurs).
The above implies the diagnosis of Sauropterygia (all taxa closer to
plesiosaurs/elasmosaurs/pliosaurs and nothosaurs than to other
lepidosauromorphans) consists of 6 characters:
"Postorbital skull elongated; upper fenestra enlarged; "euryapsid"
(lower temporal arch lost); elongate neck (>7 cervicals); curved humerus;
radius equal length to ulna."
*Mesosaurus* (1) lacks the upper temporal fenestra; (2) has a very short
postorbital cranial region, shorter than the height of the rear of the
skull (which is almost certainly about as wide as it is deep unless
compressed skeletons exagerrate the width by flattening the skull, a which
point it's even narrower than most photos show); (3) possesses the lower
(non euyrapsid, non-anapsid, but oddly synapsid); (4) has an short neck
comprised of numerous vertebrae (almost half that of *Lariosaurus*); (5)
has a humerus with straight cranial margin and curved caudal margin; (6)
has an ulna and radius which are subequal.
Rieppel and Wild (1996) diagnosed some basal sauropterygians
(nothosaurs, including *Lariosaurus*) by: (1) long snout, small, narrow
head [_]; (2) pterygoid vacuities closed by extension of pterygoid [/];
(3) extreme dorsoventral flattening of temporal region with the
development of occipital flange on the squamosal and parietal, allowing
posterodorsal expansion of the jaw adductor musculature [_]; (4) long neck
[_]; (5) unique pectoral girdle: essentially circular in horizontal plane
with large medial opening [_]; (6) scapular blades greatly reduced [*] and
extend below clavicle [_]; (7) distal limb elements reduced [*], humerus
stout [_]; (8) large coracoid with posterior extension (back stroke of
anterior limbs?) [*]; (9) pelvic girdle somewhat similar, but more
[*] indicates a presence in *Mesosaurus*; [_] that character is not
present in *M.*; [/] indicates I cannot verify this feature.
As discussed before, *Mesosaurus* shares with some few other sauropsids
the presence of an elongated fifth (outermost) digit; it also possesses a
manus/radius nearly half the arm length, a manus less than 1/3 the arm
length, pachyostatic ribs (as in whales, manatees, and marine snakes such
as seasnakes and the large swimming "legged" snakes of the Near East), a
slender (but not over long neck), an elongate rostrum....
Nearly all of these features are seen in choristoderes (as in
*Champsosaurus*), thalattosaurs (as in *Endennasaurus*), in nothosaurs (as
in *Nothosaurus,* *Lariosaurus,* etc.), in pachypleurosaurs (as in
*Pachypleurosaurus*) and so forth. None of these animals forms a natural
grouping, nor do I recall there being a suggestion where more than two of
these ever did. Müller, in his PhD thesis, conducted an analysis that
provided these above facts in separating taxa and is currently the most
thourough treatement of thalattosaurs to date, part of which was published
in Müller, Renesto & Evans (2005: The marine reptile *Endennasaurus* from
the Upper Triassic of Italy, _Palaeontology_ 48(1):15-30). He used
*Saurosternon,* a mesosaur, and found it in a group with *Palaeagama*, and
they were grouped with *Coelurosauravus*:
| `--+--Saurosternon <----
| `--Hupehsuchus <----
| |--Endennasaurus <----
| `--+--Clarazia <----
| `--Thalattosaurus <----
| `--Rhynchocephalia <----
Arrows indicate all the elongate-bodied, small-headed, long-necked,
aquatic, sometimes marine, members of Sauropsida (some, such as
*Hupehsuchus* are not as applicable as others). The tree is composed of
the concensus of 4 most parsimonious trees using 182 characters, has 874
steps, has a consistency index of 0.4, and a heuristic index of 0.75.
Müller's reasons are as follows (pg.131-132):
"The clade including *Coelurosauravus*, *Palaeagama*, and *Saurosternon*
is unequivocally diagnosed by the superficial attachment of the teeth
(#38, ci=0.429), the posterior termination of the postorbital prior
to reaching the caudal limit of the parietal (#131, ci=0.143), the
presence of distinct trochlea and capitellum on the humerus (#146,
ci=0.6), the long metacarpal IV relative to metacarpal III (#148,
ci=0.25), and the straight fibula (#149, ci=0.4).
"Implementing ACCTRAN character optimization yields as additional
synapomorphies the prominent epicondyles (#63, ci=0.444), the deep
intertrochanteric fossa (#71, ci=0.375), the marginal position of
external nares (#85, ci=0.125), the dichocephalous trunk ribs
(#104, ci=0.333), the elongated caudal projections (#107,
ci=0.286), and the contact between maxilla and prefrontal (#179,
"DELTRAN character optimization adds support to the node by the absence
of posterolateral frontal processes (#10, ci=0.25) and the distally
tapering haemal spines (#108, ci=0.462).
"*Palaeagama* and *Saurosternon* share the following unequivocal
synapomorphies: metatarsal V is shorter than the remaining metatarsals
(#79, ci=0.25), a mineralized sternum is present (#81, ci=0.2),
and the supinator process is confluent with the femoral shaft (#176,
"Additional synapomorphies under ACCTRAN character optimization are the
small premaxillae (#1, ci=0.2), the restriction of the squamosal to
the dorsal region of the cheek (#18, ci=0.3), the lack of an
process of the quadratojugal (#19, ci=0.286), the presence of dorsal
intercentra (#42, ci=0.6), the presence of a zygosphene-zygantrum
articulation (#44, ci=0.5), the absence of slender and tapering
cervical ribs at low angle to the vertebrae (#102, ci=0.167), the
absence of an ascending process of the maxilla (#126, ci=0.4), the
absence of an orbital exposure of the maxilla (#128, ci=0.444), and
the well-defined astragalus/distal tarsal IV articulation (#150,
"Implementing DELTRAN character optimization, additional synapomorphies
are the T-shaped interclavicle (#55, ci=0.375), the deep
intertrochanteric fossa (#71, ci=0.375), and the elongated caudal
lateral projections (#107, ci=0.286)."
Müller also noted (pg. 147) "It is not the first time that
*Coelurosauravus*, *Palaeagama*, and *Saurosternon* are shown to be
closely associated (see also Caldwell, 1996), even though the overall
morphology of these forms looks superficially quite different.
Unfortunately, there are no unambiguous synapomorphies (ci=1), and, as
indicated above, the exclusion of *Apsisaurus* weakens the stability of
the node. Personally, I think that the close affinity of the three taxa
should not be considered to be equal to a relationship like, e.g., between
rhynchocephalians and squamates. The reason is that the monophyletic
grouping may reflect a widespread early radiation of some diapsid reptiles
of which we know only a few members today, and that these forms, although
still monophyletic, would turn out to be widely separate if additional
taxa were known. As most convincing synapomorphy of the clade one may
regard the superficial attachment of the teeth, a trait that is not
widespread among diapsids and in the present analysis is only shared by
lepidosaurs and *Clarazia*."
Caldwell, M. W. 1996. Ichthyosauria: A preliminary phylogenetic analysis
of diapsid affinities. _Neues Jahrbüch für Geologie und Paläontologie,
One will note that the comment about tooth implantation is given some
weight: mesosaurs are not thecodont, whereas sauropterygians are. Also
note the pelvic anatomy: ilium is slender and almost rod-like, with a
short pubis, not at all broad and plate-like, and the slender elements of
the shoulder and pelvis bely the condition seen in sauropterygians.
The above should also cover some issues of the temporal region.
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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