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Mesonychians part II: Andrewsarchus was a hell of a lot weirder than all the books say

Category: mammalogy
Posted on: August 11, 2009 5:54 AM, by Darren Naish

The previous article was a brief, cursory introduction to the mesonychians. Time to look at things in a bit more detail...

Andrewsarchus_DB_wikipedia.jpg

Andrewsarchus mongoliensis is, of course, 'the' mesonychian for most people, and one might get the impression that it's a typical member of the group. In fact it's most definitely not typical, and - ironically - it's not even part of Mesonychia in the majority of recent phylogenetic studies (O'Leary 1999, O'Leary & Geisler 1999, Gatesy & O'Leary 2001, Geisler 2001, O'Leary et al. 2003, O'Leary & Gatesy 2008)... though this might be due to the fact that it's pretty poor known (only its skull can be coded for). Van Valen (1978) regarded Andrewsarchus as an arctocyonid (like so many Paleogene mammals, arctocyonids are of uncertain affinities - they've traditionally been regarded as condylarths [see previous article] - but they don't seem closely related to mesonychians), and Gingerich (1998) implied that this is at least plausible. Anyway, it owes its fame to its large size (its skull is 83 cm long), and - importantly - to the fact that its description was published in English, and in a relatively accessible publication (the AMNH's in-house journal American Museum Novitates; note, however, that the description in question (Osborn 1924) is annoyingly brief and mostly devoid of detailed morphological information). These facts have of course guaranteed its place in every single prehistoric animal book [adjacent image from wikipedia; by Dmitry Bogdanov].

Andrewsarchus_NHM_cast_9-8-2009.jpg

Indeed, what can I say about Andrewsarchus that hasn't been said before? Not much. Osborn's paper includes the interesting observation that the holotype Andrewsarchus skull was thought for a while to be that of an entelodont and, as he noted, the two were at least superficially similar, perhaps due to similar omnivorous feeding habits (though note that the possibility of a close phylogenetic relationship between Andrewsarchus and entelodonts is real: see O'Leary & Gatesy (2008)). Szalay & Gould (1966) expanded on this, noting that the semicircular arrangement of the premaxillary teeth and premolar morphology is very similar in the two, and they even noted that the teeth of Andrewsarchus would have been identified as those of an entelodont if found on their own.

Andrewsarchus_Osborn_1924_Fig_2_9-8-2009.jpg

While it may well be that Andrewsarchus was an entelodont-like predator and/or omnivore (for more on entelodonts see Tet Zoo picture of the day # 23 and Giant killer pigs from hell), there are, however, indications that Andrewsarchus was freakishly weird, and doing something very special. In fact, the weirdness of its skull is routinely not depicted correctly in life restorations. Its snout is strikingly long and narrow, and 'pinched in' about half-way along its length. The result is that the distal end of the rostrum forms a sub-circular rosette that almost resembles that of a spinosaurid theropod. Its orbits are located way down on the sides of its skull and were widely separated by the broad base of the snout, and its entire occipital region looks narrower, and smaller, than expected in an animal of this size. The glenoid fossa is flattened compared to that of 'proper' mesonychians, and the associated pre- and post-glenoid structures on the zygomatic arch are small. The sagittal crest is small as well. These features all suggest that the jaws were relatively weak. And while the upper canines were said by Osborn (1924) to be enormous, they aren't: 'The canines are very reduced in size and in proportion to the whole dentition and to the whole skull' (Szalay & Gould 1966, p. 154). Clearly, Andrewsarchus is just crying out for some neat functional study on cranial kinematics and function. Come on: surely someone, by now, must have thought of doing FEA or something on its skull?
http://scienceblogs.com/tetrapodzoology … art_ii.php
More mesonychians next: Andrewsarchus and the triisodontines.

Выходит синоним эндрюзарха - паратриизодон...
Andrewsarchus mongoliensis Osborn, 1924 (= Paratriisodon henanensis Chow, 1959)

"А как недавно выяснилось, эндрюзарха-то и триизодонтидом, и даже мезонихией назвать весьма и весьма спорно."
То, что он мезонихия, это вроде точно.

Арктоциониды сейчас вроде выделены из кондиляртр (наконец-то этот "таксон" начали разбирать потихоньку) и вполне могут быть близки мезонихиям и эндрюзарху в том числе.

К отряду арктоционий. Без шуток)).

Отредактировано Ergaster (08 September 2009 21:30:27)

Ergaster :

К отряду арктоционий. Без шуток)).

Отряд Арктоционии или Прокреоды http://www.helsinki.fi/~mhaaramo/metazo … reodi.html

Весьма интересно :

"CONVERGENT FEATURES OF Andrewsarchus
AND ENTELODONTS

Osborn (1924) briefly referred to the convergent
features of Andrewsarchus and entelodonts.
He pointed out that the cranial
and facial proportions in the giant mesonychid
are strikingly similar to those of Entelodon
and Dinohyus (p. 1) " . . . doubtless because
of similar omnivorous feeding habits."
In addition to Osborn's remarks, it is of interest
here to point out some additional
similarities of and differences between Andrewsarchus
and entelodonts that can be inferred
from the skull. The portion of the
muzzle anterior to the canines is very similar.
Both in entelodonts and in Andrewsarchus the
incisors are arranged in the premaxilla in a
semicircular pattern. The PI is usually double
rooted in entelodonts but single rooted in
Andrewsarchus and the known mesonychids.
The second and third upper premolars of
Andrewsarchus, were they known only as
isolated teeth, would certainly be allocated
to the Entelodontidae. Both in Andrewsarchus
and entelodonts the first and second
upper molars are much more heavily worn
than either P4 or M3.
Andrewsarchus is probably the most aberrant
genus in the family Mesonychidae. As
is strongly suggested above, its feeding mechanism
was much more entelodont than
mesonychid-like. Below, we briefly point out
the cranial characters of Andrewsarchus that
diverge from those of the rest of the Mesonychidae.
The sagittal crest (and very probably
the temporalis muscle) are much reduced and
the glenoid fossa is more flattened, with a
less-developed preglenoid process and postglenoid
process. These characters alone, since
we lack any knowledge about the mandible,
might indicate a weakened bite in Andrewsar-
chus, as compared to the majority of mesonychids.
The heavily wrinkled crowns of the
cheek teeth suggest an ominvorous diet also.
The functional significance of the reduced
canine and enlarged 12 is not clear. It can
mean either a reduction of the canine function
or a shift anteriorly of the area of caninization.
These two explanations appear to
be mutually exclusive, but they may not be.
Osborn (1924) reconstructed height and
other size measurements of Andrewsarchus,
based on the proportions of Mesonyx. Such a
conjecture seems unjustified, since the entelodont-
like mesonychid giant is certainly
very different from other mesonychids in
habitus and consequently probably also unlike
in body proportions. If a size estimate
must be made of Andrewsarchus, then body
proportions of entelododonts would be more
profitable to follow than those of other mesonychids."

В пропорциях тела значит надо ориентироваться вовсе не на мезонихий... Всё это конечно спорно, т.к. сходство в строении черепов необязательно должно указывать на сходные пропорции тела. Но принять к сведению можно.

женьшень :

Эндрюзархус
http://s42.radikal.ru/i097/0909/54/9d0363aeb980.jpg

Мяукнул конь !
И кот заржал !
Сие творенье увидав .

Оригинальное видение .

И "закос" под антилопу)) Резвый какой! Сколько художников - столько энрюсархов:

Ergaster :

И "закос" под антилопу)) Резвый какой! Сколько художников - столько энрюсархов:
http://s52.radikal.ru/i137/0909/cd/bb79bd26b1c4t.jpg

Неплохая реконструкция , жаль Звероящер свою реконструкцию незакончил,уверен она была бы лучшая !!!!

Да каждый, по ходу, во что горазд. Вот вроде очень хороший художник Карл Буэлл закончил в августе сего года собственное видение эндрюсарха. На изображении видно, что череп был очень широким, а что был еще и очень длинным, не видно ни разу((:

Инфа по эндрью на ,,Тетраподзоологии,, + новая реконструкция:

http://scienceblogs.com/tetrapodzoology … archus.php