Marsupial "Primates"

Dear Constant Readers,

Living in the United States creates the impression that the Virginia Opossum (Didelphis virginiana) is some sort of interloper from marsupial-infested Australia. Opossums are members of the family Didelphidae and order Didelphimorphia which include at least 60 species (numbers vary between sources). Didelphids are currently restricted to South and North America but in the past ranged to Africa, Asia, Europe, and Antarctica - the latter implying that they might have made it to Australia (there is no fossil proof of this to my knowledge). Phylogenetically they are placed at the very base of the marsupial supertree according to Cardillo et al 2004; most closely related are the 6-7 species of New World "shrew opossums" of the family Caenolestidae (Order: Paucituberculata). It goes without saying that the "possums" (Suborder Phalangeriformes) of Australasia are not directly related. Interestingly there is a third order of marsupials (Microbiotheria) from the New World that is grouped within the Australasian marsupials - a rather amazing phylogenetic situation. So with scores of species and 3 of 7 orders, the New World isn't a very place for marsupials after all.

Rather than continue with an overview of the order, I am going to focus on one group in particular, the woolly opossums. The subfamily Caluromyinae (Glironia, Caluromysiops, Caluromys) is shown as being basal in the didelphid family tree; typically it would be called "primitive" but this is misleading. Remarkably, as the title suggests, this group show convergences upon the primates. As noted by this journal which I unfortunately cannot access, the genus Caluromys shows features such as a large brain and eyes, small litters and slow development, and agile locomotion. Rather unexpected from a relative of the fairly humble Virginia opossum! Science has progressed from the days of large brained animals being more "advanced" or "better" than ones that are not - there are reasons for animals not all "aspiring" to be anthropomorphic. With that out of the way, the question is, why do these marsupials resemble primates?

The woolly opossum Caluromys derbianus. Photo taken from this page and credit is due to Maya Nature by Thor Johnson. Note the resemblance to a prosimian such as, say, a mouse lemur.

Lemelin et al 2003 note that didelphids prefer a wide variety of habitats and the genus Caluromys spends most of its time in the canopy on thin branches. The hands and feet of this genus have longer toes and the hind limbs show a much more prominent big toe. The study in the paper demonstrated that while most mammals use a lateral sequence for walking (Right hand, RF, LH, LF) most primates use a diagonal sequence (RH, LF, LH, RF) while in arboreal settings. The paper supports the theory that an arboreal habitat in early primates shaped the gait and specializations such as the hindfeet by showing that woolly opossums display the same traits.

Aside from the resemblance to what an early primate may have looked like, members of the family Caluromyidae are interesting in their own right. Oh wait, did I say family? Sources disagree if all modern opossums are all members of the same family or not; the diversity of opossums (extinct or otherwise) in one family does seem rather high but I haven't heard anything definitive. I'll just vaguely refer to them as caluromyids or woolly opossums to sidestep this issue. Walker's Mammals of the World sets the genus apart from other didelphids by noting characters such as partially derived ankle features, the retention of a cloaca, an ossified bony palate, and inner ear structure. Interestingly, this group of opossums are distinguished for having retained a pouch whereas the 50+ species of the (Paraphyletic? Polyphyletic?) marmosid group lack one. A marsupial without a pouch, hmm.

Glironia venusta, the "bushy-tailed opossum". Illustration taken from here. I'm not sure what language that is in... (Edit: Apparently Maltese)

Anyways, generally placed with the caluromyids (Cardillo et al 2004) or sometimes in its own family (Nowak, 1999) is the species Glironia venusta. It is similar to a genus of the aforementioned pouchless opossums (Marmosa) and is believed to be largely arboreal. There was a report mentioned in Walker's of this species leaping from vine to vine and apparently hunting insects - behavior not known with any other opossum species. Interesting though this species sounds, there have only been 9 specimens documented so information is a tad scarce.

Caluromysiops irrupta is another rare genus of caluromyid, apparently quite closely related to Caluromys. The species inhabits humid forests and is believed to arboreal and nocturnal; it feeds on nectar and probably has a comparable omnivorous diet to Caluromys. The familiar Virginia Opossum rarely exceeds a 3 year lifespan (record of 5) but this species, rarely kept in captivity, has a current record of 7 years and 10 months. But again, information on this species is lacking.

Caluromys laniger. Taken from this page (in Italian). One of the few pictures no explicitly copyrighted and it demonstrates arboreal movement to boot!

The genus Caluromys is of course the best known of the group, there are well over a dozen journal articles that I casually found on this genus alone. The species has been kept in labs and there are articles on hormone levels and nitrogen requirements for this genus, for instance. A pretty definitive article can probably be written on this genus but that is of course outside of the scope of this post, I only have so much time here. I will still try and concentrate on the big picture here though. This genus also drinks nectar from flowers and is believed to be a pollinator (Gribel, 1988) however it is known to eat other plant and animal material (Bucher and Hoffmann 1980). Caluromys species also has a long lifespan (6 years 4 months is the record) a small litter size (2-4 average) for an opossum and a near-exclusive arboreal habitat (Nowak, 1999). Could those also be viewed as convergences with the primates? C. philander and C. lanatus are fairly common and listed as near threatened by the IUCN Red List while C. derbianus is a vulnerable species. Glironia and Caluromysiops are listed as vulnerable as well (I'm surprised Glironia isn't data deficient).


I knew I could write a post on terrestrial animals! It wasn't much of a challenge really, like most of my post it has been floating around in my head for a few months. Sigh. Things are about to get very busy again, so who knows what I'll post next. A speculative post on opossum-men? Nah.

-Cameron


References:

Bucher, John E. and Hoffmann, Robert S. 1980. Caluromys derbianus. Mammalian Species 140, 1-4. Available: Here

Cardillo, Marcel et al. 2004. A species-level phylogenetic supertree of marsupials. J. Zool., Lond. 246, 11-31. Available: Here

Gribel, Rogerio. 1988. Visits of Caluromys lanatus (Didelphidae) to Flowers of Pseudobombax tomentosum (Bombacaceae): A Probable Case of Pollination by Marsupials in Central Brazil. Biotropica 20, 344-347. Available: Here

Lemelin, Pierre et al. 2003. Footfall patterns and interlimb co-ordination in opossums (Family Didelphidae): evidence for the evolution of diagonal-sequence walking gaits in primates. J. Zool., Lond. 260, 423-429. Available: Here

Marshall, Larry G. 1978. Glironia venusta. Mammalian Species 107, 1-3. Available: Here

Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th edition. Johns Hopkins University Press

The Many-Finned Brochoadmones

Dear Constant Readers,

There are a few fish out there with an unusual number of dorsal fins; bichirs and reedfish are basal Actinopterygians with a profusion of dorsal fins; some members of the family Scombridae (derived Actinopterygians) such as tuna and mackerel possess dorsal and anal "finlets". Nauen and Lauder 2001 note that during a stroke, finlets (with very similar morphology) showed a considerable difference in bending and appear to direct water flow and increase thrust. This abstract of a recent paper I couldn't access notes that finlets do not significantly reduce drag and enhance thrust in normal swimming. It is possible (the authors note) what difference they did cause either was meaningful over long periods of time or more useful at high speed. I haven't seen any discussion on what the mechanics of the many dorsal fins on a bichir are; since their lifestyle is pretty different from the super-performing scombrids I'm guessing they're not homologues. I've noticed that sturgeon have pre-dorsal scutes, but they may not play a role in swimming.

The butterfly kingfish (Gasterochisma melampus) showing dorsal and anal finlets. A public domain image from Fishbase by Robbie N. Cada.

A nice and old (therefore public domain) image of the bichir, Polypterurs bichir

These fish are, of course, not the focus of this particular blog post. They do illustrate that fish mechanics, particularly dealing with unconventional fin setups, are not as well understood as I anticipated. What then of a weird and obscure species from a weird and extinct group? Acanthodians, or "spiny sharks" are a radiation of jawed fishes distinct from the placoderms, chondrichthyans (sharks, skates/rays, chimeras), and bony fish. Acanthodians don't seem to have very much written on them at all; they're more closely related to bony fish than other groups (they're both Teleostomians), but their scales are more similar to those of chondrichthyans; and same species have ventral projections ("spines") appearing like multiple fins. All other fish have 2 paired fins, but some Acanthodians had many more (up to 6 spine pairs). While belonging to a unique group within Acanthodii, the recently re-described Brochoadmones sheds a lot of light on their bizarre anatomy.

Brochoadmones was a Devonian (430-435 mya) fish described all the way back in 1977. It had been demoted to a known Acanthodian family in '79 was put in its own sub-order in 1996 based on more evidence. As illustrated in Hanke & Wilson 2006, the reconstruction changed from a generalized fish with multiple finlets in 1977 to a humped salmon-like fish with shark-like gills. With two beautifully preserved fossils in the "Wonder Block" this fish is now almost completely know, and boy is it strange looking:

There was a photograph also used by Fish Feet, but at some point of time I'm going to make my own illustration based on the fossil. Note the shark-like gill slits and a pronounced pectoral fin. That last one is important...

The overall body shape of Brochoadmones is somewhat reminiscent of knifefish, the panther grouper, and a couple cichlids. These fish are stealthy ambush predators approach prey head-on facing downwards and capture them with a short lunge. Fish were visible inside some specimens, leaving no doubt that these were also piscivores. The authors speculate that if the pre-pelvic fins could move, then perhaps they aided subtly aided in stealth. Although not mentioned directly by the authors, I suppose it is implied that the multiple fins are analogues to the elongated anal fin of the knifefish and perhaps grouper. However, the authors note that other acanthodians were built for continuous swimming and some of them also have the pre-pelvic fins as well. Did pre-pelvic fins offer any advantage over the setup of modern fish groups? Over continuous fins in some instances? Why these fins were only present in this one group is apparently not clear and will likely remain so since the peculiar (non-analogous) multiple fins of scombrids and bichirs also have no clear function.

Brochoadmones had many features aside from the number of pelvic projections that made it unique and quite interesting. The projections themselves consist of six pairs of pre-pelvic spines and a web of skin with scales covering both sides. According to the authors, that feature has not been seen in any other vertebrate and look like expectations of what a fin evolving from a lateral fin-fold would look like. The largest spine is larger than that of the actual pelvic fin spine, although the attached fin of the latter is larger in area. Also noteworthy is that the pre-pelvic spines continue quite a ways closer to the head than the pectoral fin. There have been suggestions that both types of fins evolved from an ancestral lateral fin-fold, but the authors suggest that the radically different placement and development of the fins suggest different origins (both predating jaws). So there you have it, a peculiar and obscure fish that somehow has implications for the origins of limbs. The fusion of the anal and caudal fin (along with paddlefish development) suggest that the median fins actually do develop from an ancestral median dorsal/caudal/anal fin.

Other strange features hold implications for Acanthodian phylogenetics, but I'd say that is out of the focus of this current blog post. I want to be able to blog and still do homework, dangit. Recently it seems that I have been focusing quite a bit on aquatic creatures so I think next post it is time for me to branch out a little. Well, I really don't plan these out in advance, so I suppose the next thing that strikes my fancy will come up.


-Cameron

References:

Nauen, Jennifer C. & Lauder, George V. 2001. Three Dimensional Analysis of Finlet Kinematics in the Chub Mackerel (Scomber japonicus). Biol. Bull. 200, 9-19. Available: Here

Hanke, Gavin F. & Wilson, Mark V. H.. 2006. Anatomy of the early Devonian Acanthodian Brochoadmones milesi based on nearly complete body fossils, with comments on the evolution and development of paired fins. Journal of Vertebrate Paleontology 26 (3) 526-537. Available: Here




Tangent:


Should I even mention this thing? Brochoadmones also had pre-pelvic appendages extending anterior to the distinct pectoral fins as well as two dorsal fins and a somewhat humped profile. I'm not suggesting that there are giant whale-like acanthodians out there; but perhaps this bizarre Renaissance drawing was done with an Acanthodian in mind (like griffins from Protoceratops). The modern reports may have been influenced by this myth and illustration, but the sightings primarily seem to be of cetacean pods with a dash of polychaete thrown in.

Fossil Octopodes

Dear Constant Readers,

As mentioned previously in the last post, this is an expansion of a subject not easily crammed in as a tangent. Sure there are only three examples of fossil octopodes (and some indirect evidence), but the story is shockingly complicated. As Persuasive and Analytical Writing has taught me, I really do have a tendency towards unnecessary broadness. Combating that will also probably give me a more respectable post output. So let us explore this little knot of a topic.

On an ironically placed tangent I should note the suspiciously changed title of the previous post. As far as the plural of "octopus" is concerned; "octopi" is hyper-corrected (note: pus/pod not present); "octopuses" is strange sounding and anglicized; and "octopodes" appears to be the proper plural. It is also the most international way of putting it, or so Wikipedia says to me. I try not being a typical 'murrican, I really do. All three (and "octopods") confusingly appear in recent scientific journals but from now on: octopodes.

With that nitpicking aside, yes, octopodes have a terrible fossil record. Way worse that choristoderans even. Despite having the advantage of extant species (and thus testable genetic material), the phylogenetics of "Octopodiformes" (octopodes and vampire squid) are currently quite confusing. Recent genetic analysis (Strugnell et al 2006) dealing with divergence times concluded that Octopoda and Vampyromorpha diverged some time in the Paleozoic; a diagram in the paper showed it as being in the Permian over 250 mya. As noted in the prior post, cirrate and incirrate octopodes diverged in the Jurassic, much earlier than anticipated. The paper also illustrated the major Decapodiform lineages diverging all the way back in the Devonian, perhaps around 400 mya. It should be noted that this test used examples from the fossil record in order to establish minimum ages for certain lineages and divergences. This paper did not estimate estimated the divergence of the major cephalopod divisions of nautiloids, Decapodiformes and Octopodiformes, nor did any other I could find. Things seemed peachy until...

An even more recent study by Yokobori et al 2007 discussed the relations of those major groups. Traditionally, the concept of Octopodiformes (= octopodes + vampire squids) has been morphologically supported but some genetic tests have concluded that Vampyromorpha is more closely allied with Decapodiformes. This reveals the problem with Strugnell et al 2006 of presuming the monophyly of the two traditional coleoid groups. The Yokobori et al study concluded that octopodes and vampire squids did have an mt organization closer to the proposed ancestral cephalopod condition the Decapodiformes. Interestingly, this does not support the notion that octopodes and vampire squids are necessarily sister groups. Yokobori et al conclude that there need to be three major divisions of coleoid cephalopods (Octopoda, Vampyromorpha, Decapodiformes) and that they all diverged rapidly at an ancient event before the Decapodiform radiation. So judging by the apparent Devonian (at least Carboniferous) radiation of Decapodiformes, this has to be an ancient radiation occurring relatively early in the history of cephalopods.

Now, how does this compare to what is known about fossil octopodes?


Pohlsepia mazonensis
Kluessendorf & Doyle, 2000

Most safely described as a coleoid cephalopod from the Pennsylvanian (~300 mya) Mazon Creek fauna of Illinois. Preservation of soft body parts from other cephalopods (and maybe one other specimen) are found in this formation, as is the famous and bizarre Tully Monster (Tullimonstrum gregarium). Despite these spectacular fossils, soft body preservation is still rare even in a formation such as this. The specimen is tiny (max body with 3.5 cm/1.4") with a flattened sub-circular body, two small fins, and small black eyes. The body is sac-like with an ill-defined head, characteristics typical of octopodes. Curiously the specimen does not possess 8 limbs, but has 2 additional arms longer than the rest (modified arms or tentacles). Decapodiformes modify/lose arm pair IV while Vampyromorphans and Octopodes modify/lose arm pair II, but it is never stated which arm pair is modified. This is where we begin to run into trouble. After some speculation that this may be a cuttlefish-like Decapodiform, the authors suggest that it is some sort of octopus, and compared it to cirrates. They classify it as ?Cirroctopoda, a suggestion I don't really buy. This would make cirrates paraphyletic (with incirrate and vampire squid offshoots), twice as old as indicated by genetic evidence, and would mean they re-evolved a shell somewhere along the line. Since the fins of cirrates use the shell as a muscular anchor, I was wondering if the "gut trace/ink sac" may have actually been a shell/gladius of some sort. The authors don't comment on and probably did not take into account the internal structures of "Octopodiformes". So what is Pohlsepia? I think that given the extreme separation of this fossil from any relatives and the apparent closeness to the Octopoda/Decapodiform/Vampyromorpha divergence this cephalopod may not be classifiable in any crown groups. I wouldn't give my opinion too much weight - but I think that more genetic and fossil evidence is certainly needed for this incredibly murky time period.


Proteroctopus ribeti

I tried avoiding the other fossil octopus article (here) but since I can't find the article for this species I guess I'm going to have to consult it. There certainly isn't anything wrong with the article, I just wanted to read things and put them in my own words. This specimen is from the mid-Jurassic (132 mya) of France, a time period near the divergence of cirrates and incirrates according to Strugnell et al 2006. Eyden's excellent article mentions that this specimen has suckers, a sac-like body, 8 equal appendages and fins. Strugnell et al 2006 notes that there was no shell present and they mentioned the possibility that this was a "teuthid" or stem-octopus. Eyden mentions that some authorities consider it a vampyromorph. While this has some superficial cirrate traits, the lack of a shell and cirri once again seems to be a considerable problem. Like Pohlsepia, with all traits considered it really isn't very clear what this specimen is.


Palaeoctopus newboldi

This is a Late Cretaceous (89-71 mya) octopus which bears strong resemblance to modern cirrate species. It has an indistinct head, sac-like body and 8 arms with suckers, fins, internal U-shaped fin supports and an ink sac (the last two from Voight, 1997). Judging from these characters, I would agree with the classification of this as a Cirrate/Cirroctopoda in the family Paleoctopodidae. The time period also strongly hints at this being a cirrate (as opposed to a basal Coleoid), although the ink sac hints at it not being as adapted towards a deep-water environment. While probably fairly basal in Cirrata, this is the least ambiguous fossil by far.

Table of Prominent Characteristics

	Shell/Gladius	MA	DH	Ink sac	Fins
Pohlsepia		x	-		x
Proteroctopus		-	x		x
Palaeoctopus	x	-	-	x	x
Cirrata	x	-	-	-	x
Incirrata	-	-	-	x	-
Vampyromorpha	x	x	-	-	x

MA = Modifed arms
DH = Distinct head

Argonauts

Before I leave, I should mention argonauts, usually listed separately from octopodes but certainly derived from them. While most species in the superfamily don't, in a few species the females secrete an egg case from their arms which bear an uncanny resemblance to ammonite shells. Obinautilus is the earliest known fossil argonaut from the Oligocene of at least 29 mya. Strugnell et al 2006 estimate that argonauts originated some time in the Jurassic, not much after the cirrates and incirrates diverged. This indicates that they actually did live at the same time as ammonites and may give hints as to the nature of their bizarre "shell". While not direct preservation of octopodes; Izumonauta, Kapal, Mizuhobaris, and Obinautilus certainly are the majority of fossil octopus species. Darnit, one of these days I'm going to have to do another tangent post (and one for Vampyromorphans too).



So there have it, a post on fossil forms of some of the least likely animals to be fossilized. This certainly is a rather confusing subject and unfortunately there is no definite word to be had on it. This goes to show that while books and webpages may write up some definite phylogenetic relationships, behind the scene there seems to be an immense amount of argument. With the perfection of genetic testing and the finding of additional specimens, the murky origins of octopodes may yet be discerned. Or not.


-Cameron


References:

Kluessendorf, Joanne & Doyle, Peter. 2000. Pohlsepia mazonensis, an early "octopus" from the Carboniferous of Illinois, USA. Palaeontology 43 (5) 919-926. Available: Here

Strugnell, Jan et al. 2006. Divergence time estimates for major cephalopod groups: evidence from multiple genes. Cladistics 22, 89-96. Available: Here

Voight, Janet R. 1997. Cladistic Analysis of the Octopods based on anatomical characters. J. Moll. Stud. 63, 311-325. Available: Here

Yokobori, Shin-ichi et al. 2007. Mitochondrial genome structure and evolution in the living fossil vampire squid, Vampyroteuthis infernalis, and extant cephalopods. Molecular Phylogenetics and Evolution 44 (2) 898-910. Available: Here

Cirrate Octopodes

Dear Constant Readers,

Time management is a skill I can stand to improve upon. It never ceases to amaze me how an hour can fly by without me actually doing anything productive. And now I look back and realized that I haven't posted anything on here in two weeks! Surely I can't be that busy. Of course, I have no actual obligation to post here aside from a continuous gnawing sense of guilt...

In an attempt to alleviate that guilt, I'd like to introduce the cirrate octopodes. Along with other groups such as, say, mesoplodonts, these are among my (increasingly numerous) favorite animals. Predictably for an animal I like, cirrates are publicly poorly known and anatomically bizarre compared to the much more familiar incirrate octopodes. The namesake of cirrates are filament-like cirri of variable length and development paired with the suckers. Since these are not always readily visible, I'd suggest the easiest way to differentiate a cirrate from incirrate is that the former has a pair of fins used in lieu of jet propulsion.

From the Friday Cephalopod at Pharyngula taken in turn from The Deep by Claire Nouvian. It was mis-identified as a deep-sea "squid".

These may seem like rather bizarre characteristics but they are actually ancestral and apparently quite ancient. A third group of Octopodiformes, Vampyromorpha or vampire squids, diverged from the cirrates and incirrates some time in the Paleozoic (250+ mya) according to a recent molecular analysis by Strugnell et al 2006. As demonstrated by the excellent Tree of Life webpage vampire "squids" do indeed share characteristics like cirri and fins (of which they have four in one stage) with cirrates but differ in retaining jet propulsion and a fifth pair of appendages. In a crucial post for cephalopod fans by PZ Myers there is a diagram on homology indicating that the reduced/absent appendages of Octopodiformes are in fact not the same as the tentacles of Decapodiformes (squid, cuttlefish, et cetera). o is a fossil cephalopod with a modified limb pair (homology not discussed) that was mentioned by Kluessendorf and Doyle 2000. While octopodes were though to be a recent radiation, this species dates from 300 mya in the Carboniferous. It has features such as a sac-like mantle fused to an indistinct head, lobed fins, and a lack of an internal shell. The paper didn't mention it, but that last feature is problematic since cirrates actually have cartilaginous shells. Is Pohlsepia actually an Octopodiform or an unrelated squid-like creature? Strugnell et al propose cirrates and incirrates diverged many millions of years later in the Jurassic implying that either internal shells were lost recently or actually re-evolved in cirrates. The matter of Octopodiform evolution is going to have to be untangled in a future post.

While cirrates do bear resemblance to Pohlsepia and the two other fossil octopodes, they certainly are not "living fossils" or "prehistoric survivors". I'll let Darren Naish explain to you why I also dislike those terms. A 2006 paper by Collins and Villanueva appears to be one of the most definitive work on this group to date and demonstrates many remarkable characteristics. Cirrate octopodes are adapted to living in deep and cold waters and display adaptations such as a gelatinous composition, reduced gills, lost ink sac, and lost/reduced radula. The cirri of this group's namesake have not been investigated but are believed to be sensory in function. This group also appears to have lost (or never evolved) the ability to change coloration but possesses photophores capable of generating light. Males do not have the sperm transfer arm (hectocotylus) of cirrates but they do posses enlarged dorsal arms for no clear reason. The internal cartilaginous shell is variable in shape and is associated with the muscles used to drive the fins. I'd like to suggest the possibility that the internal shell re-evolved when the animals abandoned jet propulsion for fin propulsion - it would be an interesting theory to test.

The "Dumbo Octopus" Grimpoteuthis sp. taken from Wikipedia.

As far as behavior is concerned the paper mention that jet propulsion was lost due to being high energy concerns although it could possibly still be present during burst swimming. The trait of high speed is not selected for and these species get around by benthic crawling with the arms, umbrella-style drifting, swimming with fins, and medusoid contractions i.e. jellyfish-like movements. Predation is little documented but these species are known to "balloon", invert their web (also in Vampyroteuthis) and give off phosphorescence when disturbed. The cirri appear to have some function in feeding along with suckers while enveloping or entrapping prey and they appear to be used to make currents and bring prey to the mouth. Of course, this behavior is generalized for the group and different species and genera all engage in different behaviors.

The paper concludes that while leaps and bounds are being made on research in this group, there are still many unknowns. The taxonomy is in tangles and the ecology is poorly known. Cirrates may be comparatively long lived compared with most other cephalopods and there is concern as to what effect fishing may have on this potentially fragile group.

As far as information goes I've hardly scratched the surface for cirrates. Consider this a potential warning for posts to come. However, they may not come immediately...

-Cameron



References:

Collins, Martin A. & Villanueva, Roger. 2006. Taxonomy, Ecology and Behavior of the Cirrate Octopods. Oceanography and Marine Biology 44, 277-322.

Kluessendorf, Joanne and Doyle, Peter. 2000. Pohlsepia mazonensis: An early "octopus" from the Carboniferous of Illinois, USA. Paleontology 43, 919-926. Available: Here

Strugnell, Jan et al. 2006. Divergence time estimates for major cephalopod groups: evidence from multiple genes. Cladistics 22, 89-96. Available: Here