#metazoa
Species:S. clavula
Etymology: “Little Skeem,” after the Skeem family who found the type specimen
Age and Location: Middle Cambrian of Utah
Classification:Eukarya: Opisthokonta: Metazoa: Eumetazoa: Bilateria: incertae sedis
Ah, the Cambrian explosion. We’ll be spending a lot of time here. Like Pikaia,which I wrote about yesterday,Skeemellais a debated bilaterian that may be close to the ancestry of chordates. It’s a possible member of the enigmatic Vetulicolia, a clade of exoskeleton-bearing animals that consist of a boxy anterior region and a segmented tail, which might be deuterostomes, and specifically seem to be stem-group tunicates. Vetulicolians, including Skeemella, were blind and probably poor swimmers that kept close to the seafloor. Skeemella, however, has a strikingly longer “tail” than any other vetulicolian, and it apparently lacks gill slits, which are otherwise prominent in many vetulicolians. Furthermore, it has a telson and a clearly segmented anterior body. By and large Skeemellaseems more arthropod-like than any other vetulicolian, though it’s totally unclear what kind of arthropod it could be. Vetulicolians, Skeemellain particular, also resemble the microscopic mud dragons (Kinorhyncha), a group somewhat related to arthropods, within the clade Ecdysozoa. At 14 cm long, Skeemellawas large for a Cambrian animal.
IsSkeemellaa vetulicolian? It’s hard to tell, given that only one specimen is known. It certainly resembles ecdysozoans more than other vetulicolians do, which raises the possibility that it’s an ecdysozoan that converged on vetulicolians. Alternatively, it could be a vetulicolian that converged on ecdysozoans, or it could be convergent on both, or it could be proof that vetulicolians were specialized, deuterostome-like ecdysozoans.
Sources:
Aldridge RJ., Hou X-G., Siveter DJ., Siveter DJ., Gabbott SE. 2007. the Systematics and Phylogenetic Relationships of Vetulicolians. Palaeontology 50:131–168.
Briggs DEG., Lieberman BS., Halgedahl SL., Jarrard RD. 2005. A new metazoan from the middle Cambrian of Utah and the nature of the Vetulicolia. Palaeontology 48:681–686.
García-Bellido DC., Lee MSY., Edgecombe GD., Jago JB., Gehling JG., Paterson JR. 2014. A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC evolutionary biology 14:214.
Lieberman BS. 2008. The Cambrian radiation of bilaterians: Evolutionary origins and palaeontological emergence; earth history change and biotic factors. Palaeogeography, Palaeoclimatology, Palaeoecology 258:180–188.
Shu D-G., Conway Morris S., Zhang Z-F., Han J. 2010. The earliest history of the deuterostomes: the importance of the Chengjiang Fossil-Lagerstatte. Proceedings of the Royal Society B: Biological Sciences 277:165–174.
Reconstruction by Marianne Collins, from Conway Morris and Caron 2012
Species:P. gracilens
Etymology: Derived from Pika Peak, a mountain near the type locality
Age and Location: Middle Cambrian of British Columbia
Classification:Metazoa: Eumetazoa: Bilateria: ?Deuterostomia: ?Chordata
Pikaia,despite its fame as the lancelet-like primitive chordate of the Burgess Shale and over a hundred known specimens, is a bizarre and poorly known creature. Due to the difficulty of interpreting its anatomy, it isn’t even clear if it’s a chordate at all; the only certain feature it shares with chordates is the presence of segmented muscle blocks known as myomeres. However, myomeres are also known in the seemingly non-chordate Myoscolex,which may be an annelid. Pikaiahas only a hint of a possible notochord, but it is impossible to tell from fossils if this structure is a notochord, blood vessel, nerve, or something else.
Pikaiawas a strange marine animal, known only from the Burgess Shale, where it was uncommon but not especially rare. Adult Pikaiawere on average around 4 cm long, with the largest individuals approaching 6 cm. Like most swimming chordates, it was laterally compressed; it swam by undulating its body like an eel. However, its muscles were relatively simple and Pikaiawas probably a poor swimmer that kept close to the seafloor.
Pikaiahad a pair of small sensory tentacles in front of its mouth that served as its primary sense organs. It was probably a selective feeder; its small mouth and limited ability to expel water through its tiny pharyngeal slits would have precluded suspension feeding, which is otherwise the norm in early deuterostomes. Next to its pharyngeal slits, it had tiny appendages that may have served as gills.
Pikaiahad several bizarre internal organs of unknown function; the most prominent of these was a thick “dorsal organ” running the length of its body. This structure, once incorrectly interpreted as a notochord, may have been internal cuticle that served to stiffen the body. Because of this and other oddities, it is unclear in what ways Pikaiamay relate to other chordates. It may be a perfect example of a transitional form between primitive deuterostomes and chordates, or it may be a strange side branch of chordate evolution–if it’s even a chordate at all.
Sources:
Conway Morris S., Caron JB. 2012. Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia. Biological Reviews 87:480–512.
Dzik J. 2004. Anatomy and relationships of the Early Cambrian worm Myoscolex. Zoologica Scripta 33:57–69.
Lacalli T. 2012. The Middle Cambrian fossil Pikaia and the evolution of chordate swimming. EvoDevo 3:12.
Mallatt J., Holland N. 2013. Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 320:247–271.
Species:L. yini
Etymology: “Lukou Reptile,” after the Lukou Bridge (now transliterated as Lugou), in memory of the battle fought there that began the second Sino-Japanese War.
Age and Location: Early Jurassic of China
Classification:Eukarya: Metazoa: Bilateria: Deuterostomia: Chordata: Vertebrata: Eugnathostomata: Euteleostomi: Tetrapoda: Amniota: Sauropsida: Sauria: Archosauromorpha: Archosauria: incertae sedis
Lukousaurus,previously featured on@a-dinosaur-a-day, was first classified as a coelurosaur, back in the days that “coelurosaur” meant no more than “small or medium-sized bipedal archosaur,” based largely on the fact that its skull looked similar to a skull, SMNS 12352, assigned to Procompsognathus. However, since then, SMNS 12352 has been re-assigned to Crocodylomorpha, which makes it possible that Lukousaurus, too, was a basal “sphenosuchian”-like pseudosuchian. However, without a proper re-study of this fossil, it will be hard to know whether or not it can be assigned to that group, though that now seems more likely than is a dinosaurian identity. Whatever it turns out to be, Lukousauruswas definitely a small, predatory archosaur.
Sources:
Knoll F., Rohrberg K. 2012. CT scanning, rapid prototyping and re-examination of a partial skull of a basal crocodylomorph from the Late Triassic of Germany. Swiss Journal of Geosciences 105:109–115.
Young C-C. 1948. On two new saurischians from Lufeng, Yunnan. Bulletin of the Geological Society of China 28:75–90.
A somewhat speculative reconstruction from Dzik 2004
Species:M. ateles
Etymology: “Muscle worm,” after the preservation of its muscles
Age and Location: Cambrian of Australia
Classification:Eukarya: Opisthokonta: Metazoa: Eumetazoa: Bilateria: incertae sedis
This poorly-known organism is likely to be an early relative of the annelid worms, but little is known about its anatomy or affinities; it may be an Opabinia-like arthropod relative instead. Myoscolex was probably a free-swimming animal, however, and it was clearly abundant in the early Cambrian Emu Bay fauna. It had a laterally compressed body, and so superficially resembled early chordates in shape and perhaps lifestyle. It could grow up to 10 cm long, and it seems that as it grew it gained more segments or annulations. The body was lined with bristles, like living polychaetes. It had a clearly differentiated head, however, due to poor preservation we don’t know what the head looked like. It may have had rows of some kind of glands along its sides.
Sources:
Briggs DEG., Nedin C. 1997. The taphonomy and affinities of the problematic fossil Myoscolex from the Lower Cambrian Emu Bay Shale of South Australia. Journal of Paleontology 71:22–32.
Dzik J. 2004. Anatomy and relationships of the Early Cambrian worm Myoscolex. Zoologica Scripta 33:57–69.
Species:K. colberti
Etymology: “Cupped hand tooth,” after the shape of the tooth
Age and Location: Late Triassic (Norian) of Arizona
Classification:Vertebrata: Eugnathostomata: Teleostomi: Euteleostomi: Sarcopterygii: Tetrapoda:?Amniota: incertae sedis
Another bizarre tooth taxon from Triassic North America, Kraterokheirodonis among the most enigmatic members of the Chinle fauna. Once interpreted as a cynodont, and in fact featured in Walking with Dinosaurs as such, it is now considered an indeterminate amniote because its only known specimens are so hard to interpret. Whatever this animal was, however, it was large–the teeth are a few centimeters across.
Each tooth seems to consist of a row of six cups of variable size lined up perpendicular to the jaw. No known animal has a tooth that looks very much like this, though the tooth root makes it likely to belong to an amniote. It looks most like the tooth of a traversodont cynodont, somewhat similar to Exaeretodon. However, closer examination suggests that these similarities are superficial at best, so the mystery remains unsolved. If it is not a traversodont, it might be a different type of cynodont, or an unusual reptile perhaps similar to Trilophosaurus. This animal will probably remain a mystery until a lucky fossil collector finds a jawbone that contains some of these distinctive teeth.
Sources:
Irmis RB., Parker WG. 2005. Unusual tetrapod teeth from the Upper Triassic Chinle Formation, Arizona, USA. Canadian Journal of Earth Sciences 42:1339–1345.
Species:D. enigmatica (type), D. discoides (referred)
Etymology: “Dendrogram,” for the dendogram-like internal structure
Age and Location: Modern ocean southeast of Australia
Classification: Metazoa incertae sedis
Dendrogrammais relatively unique among problematic life, in that it’s still alive today. Its phylogenetic position may be resolved based on DNA if any is ever successfully extracted. It appears to be a primitive animal, similar to cnidarians and ctenophores, but it is clearly distinct from both of those groups. It is possible that it may represent a new “phylum” entirely, or that it might be the only living descendant of some group of the bizzare Ediacaran biota. However, the only known specimens are juveniles, which raises the possibility that these are simply bizarre flatworm larvae, rather than a totally new branch of the tree of life.
Sources:
Just J, Kristensen RM, Olesen J (2014) Dendrogramma, New Genus, with Two New Non-Bilaterian Species from the Marine Bathyal of Southeastern Australia (Animalia, Metazoa incertae sedis) – with Similarities to Some Medusoids from the Precambrian Ediacara. PLoS ONE 9(9): e102976. doi:10.1371/journal.pone.0102976
