Saturday, May 12, 2012

MOTHER’S DAY 2012: Celebrating Ancient Sharks, Superfoetative Viviparity, and the Earliest Record of Sibling Rivalry


Montana, 318 million-years-ago. Red line is the equator and the black line marks 10 degrees north latitude.  Yellow circle is the site of the Bear Gulch Limestone.


Montana winters were certainly nicer during the Late Mississippian period some 318,000,000 years ago. Montana was much warmer --- it was only 12o north of the equator and over half the state was covered by shallow, warm, tropical seas.  Although the skiing and snowboarding was non-existent the surfing was vastly improved.  Of particular interest to this post is an area in central Montana that during Mississippian times was a shallow bay, only 40 meters deep and 15 km long.  Today the sediments deposited on the floor of this bay are known to geologists and paleontologists as the Bear Gulch Limestone (BGL) and those rocks are world renowned for their truly spectacular fossil record . 


The fossil bearing beds of the Bear Gulch Limestone.

The Bear Gulch Limestone occurs in repeated layers that separate well so it is not surprising the fossil beds were discovered by local ranchers who were quarrying it for ornamental building stone.  Although only discovered in 1977, the BGL has yielded about 130 species of fish, some 65 of which are chondrichthyans, the “cartilaginous” fishes, such as primitive sharks, rays, and holocephalans. 

Because most of their skeleton consists of cartilage rather than bone, and cartilage has a much lower fossilization potential, a great deal of  the fossil record of sharks looks like this.....



.... little more than shed and isolated teeth.  The spectacular fossilization in the BGL, preserves not only cartilaginous skeletons but soft internal tissue as well as skin. Thus it is no wonder that the Bear Gulch Limestone is one of our best windows onto life of the Mississippian. 

And it’s because of that remarkable preservation that we turn to the Bear Gulch Limestone and two of its primitive fish to celebrate Mother’s Day 2012.  


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Harpagofututor and the Joys of a Large Family

The spotted ratfish (Hydrolagus colliei), a living holocephalan.

Harpagofututor is a primitive member of the Holocephali, a group of still living cartilaginous fishes.  They are known by a variety of popular names (rat fish, chimaeras) and most of the 34 living species inhabit deep sea environments, with a few species preferring shallow water.  Although an ancient lineage, originating some 400,000,000 years ago, holocephalans remain poorly known to the general public.


Sexual differences in Harpagofututor.  Top is a female.  Bottom is a male.  Arrows point to the head appendages characteristic of males of this species of fish.

Harpagofututor is not a particularly large fish, reaching a maximum size of 165mm (~7in).  It has an elongate, eel-like, body and is known from numerous specimens. This fish comes in two forms, one with a “normal head” and the other with two large cartilaginous appendages projecting from the top of the skull in front of the eyes. Each appendage is jointed at mid-length and there is a moveable ball and socket articulation with the top of the skull.  The end of each appendage was a long thin fleshy extension bearing numerous small hooks. The two forms are otherwise quite similar and these striking differences are due to sexual dimorphism, with the male using the appendages for display or to hold onto the female during copulation, as do some living holocephalans.

The unadorned skull of a female Harpagofututor.


The skull of a male Harpagofututor, showing the large, paired, jointed, cartilaginous appendages (colored red) that moveably articulate with the top of the skull.
However, Harpagofututor was thoughtful enough to confirm this suspicion of sexual dimorphism by having two unornamented specimens preserved with embryos inside the body cavity, confirming them as female.  Sometimes the fossil record plays nice.  

The number of embryos preserved is 4 in one specimen and 5 in the other. Within each group one embryo is distinctly larger (based on skull size), with the skull length of the largest approaches that of adults. Thus it would appear that Harpagofututor young were born live (known as viviparity) and at a large size. This would give the newborns a distinct advantage in obtaining food and surviving.

A pregnant female Harpagofututor. A. Fossil with mass of embryos in abdominal area circled in yellow.   B. Line drawing of A.   C. detailed drawing of the five embryonic skulls.  Note range of size of skulls.  Scale bar = 1 cm.


The larger individuals in each female are clearly at a more advanced stage of development than its litter mates.  This is most likely due to superfoetation, where a female carries multiple litters and the young exhibit distinctly different stages of development.  Thus what we are seeing are young that came from eggs fertilized at different times.  This might be due to separate copulations after separate ovulation events, or from a female storing sperm from only one copulation and using it to fertilize eggs from different ovulation cycles.  In some of the female specimens with soft tissues preserved, sperm receptacles are present, suggesting that female storing of sperm occurred in Harpagofututor.  

A second pregnant Harpagofututor. A. Photo of Specimen.   B.  Line drawing of A, showing embryonic skulls.  C. Photo of  largest of the embryonic skulls.  D. Line drawing of two embryonic skulls. Scale bar = 1 cm (in mm).
  
Harpagofututor is the only known case of superfoetation in the fossil record. It has evolved numerous times in boney fish but this is only the second known record for cartilaginous fishes.  The other is that of a living whale shark carrying 300 young at three different developmental stages.  

So now knowing a little bit about the morphology and biology of Harpagofututor, you can better appreciate the meaning of the name, which is derived from harpagos, grappling hooks, and fututor, copulator, in reference to the secondary sexual apparatus of the males of this species.


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Old Womb Tooth and the First Sibling Rivalry

The other star of this post is Delphyodontos  and we only know of it because of the spectacular fossilization found in the BGL. Only two Delphyodontos specimens have been described in the scientific literature, and both are very small, although larger, newly discovered specimens are currently under study.

One of the two known specimens of Delphyodontos.  Note preserved fecal sac at the back of the abdomen. Total length 29mm (1.2 in).
 
Delphyodontos is a primitive cartilaginous fish.  The two described specimens measure 35mm (1.4 inches) and 29mm (1.2in) respectively. Its shape is peculiar, with an enlarged abdomen, poorly developed fins, and an arched backbone.  These features indicate that Delphyodontos was relatively helpless and not capable of swimming. However, these features are also characteristic of a fetus. The known specimens are most likely spontaneously aborted late term fetuses.

What is even more intriguing than its body shape is its teeth. The upper and lower tooth plates are compressed, blade like, and bear tall cusps.  This tooth structure is adapted for slashing and piercing.  That's unique for holocephalans for all other known members of the group, both living and fossil, have crushing tooth plates.  Although Delphyodontos adults have not yet been described, it is highly likely that they had crushing teeth as well.  So what would a Delphyodontos fetus need such teeth for?  In addition, there is no evidence of a yolk sac or umbilicus, so what was the fetus feeding on? 

Drawing of the piercing, slashing teeth of Delphyodontos.
The answer comes from the smaller specimen, in which a large fecal mass is preserved in the back of the abdomen.  Since these small, fetal fish were not capable of swimming yet show a fecal mass in the gut, they must have been feeding inside the female before birth.  There are, surprisingly, food sources within the mother, notably other eggs. Such intrauterine feeding is known in living cartilaginous fishes, where the yolk sac is absorbed early in development and specialized dentition is used to open egg capsules and consume the contents.  In some cases, the mother shark provides a steady supply of unfertilized eggs as a food source. The fetus of some modern intrauterine feeding sharks show an expanded foregut, as in Delphyodontos.

So as peculiar as Delphyodontos looks, its features give us insight into its biology, even before birth.  And if eating the other eggs sharing the uterus with you isn’t sibling rivalry, then I  have no idea what is. 

Even though every blasted dinosaur species has been fleshed out multiple times, this is the only available restoration of Delphyodontos and shows the overall embryonic shape of the specimens.

Hopefully, knowing about the biology of Delphyodontos gives you a better appreciation of its name, which is derived from delphyo, womb, and dontos, tooth.



I wish all my readers a happy 2012 Mother's Day.  Go ahead, give mom a hug and a kiss and, if you feel like it, gobble down a couple of eggs in memory of old Delphyodontos.  

But stay tuned for my Father's Day post.  I already have it plotted out.  


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PHOTOS
Paleogeographic map,  BGL outcrop, photo of male and female Harpagofututor, and photo of Delphyodontos specimen (some modified): Fossil Fishes of Bear Gulch  http://www.sju.edu/research/bear_gulch/


Drawings of male and female skulls of Harpagofututor (modified):  Lund 1982 (below).


Two pregnant Harpagofututor specimens (modified): Grogon and Lund 2011 (below)

Drawing of jaws and teeth of Delphyodontos (modified): Lund 1980 (below)

Delphyodontos flesh restoration: http://en.wikipedia.org/wiki/Delphyodontos


SOURCES

Fossil Fishes of Bear Gulch. www.sju.edu/research/bear_culch/index.htp

Grogon, E.D. and Lund, R. 1997. Soft Tissue Pigments of the Upper Mississippian Chondrenchelyid, Harpagofututor volsellorhinus (Chondrichthyes, Holocephali) from the Bear Gulch Limestone, Montana, USA.  Journal of Paleontology, Vol. 71 (2): 337-342.
Grogon, E.D. and Lund, R. 2002. The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition.  Geodiversitas 24(2): 295-315.

Grogon, E.D. and Lund, R. 2011. Superfoetative viviparity in a Carboniferous chondrichthyan and reproduction in early gnathostomes.  Zoological Journal of the Linnean Society 161: 587-594.

Hagadorn, J.W. 2002. Bear Gulch: an exceptional Upper Carboniferous Plattenkalk. In Bottjer, D.J., Etter, W., Hagadorn, J.W., & Tang, C.M. (eds.) Exceptional Fossil Preservation: A Unique View on the Evolution of Marine Life. New York: Columbia University Press: 167-183.

Joung, S.J., Chen, C.T., Clark, E., Uchida, S.W., and Huang, Y.P. 1996. The whale shark, Rhincodon typus, is a livebearer; 300 embryos found in one ‘megamomma’ supreme. Environmental Biology of Fishes 46: 219-223.

Lund, R.  1980. Viviparity and intrauterine feeding in a new holocephalan fish from the Lower Carboniferous of Montana. Science 209 (4457): 697-699.

Lund, R. 1982. Harpagofututor  volsellorhinus new genus and species (Chondrihthyes, Chondrenchelyiformes) from the Namurian Bear Gulch Limestoine, Chondrochelys problematica Tarquir (Visnean) and their sexual dimorphisms. Journal of Paleontology 56(4): 938-958.

    

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