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Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources


Mesozoic Mammals 1 (Triassic and Early Jurassic)

        Mammals are first seen in the fossil record at the end of the Triassic period and in the Early Jurassic period, about 225-180 mya. In terms of their overall development from cynodonts, mammals are currently defined by paleontologists as the common ancestry of  all extant mammals and Sinoconodon. The latter, found in Yunnan province, China, is considered as the most basal of mammal-like cynodonts (Kielan-Jaworoska et al. 2004). Sinocodon is one of several "stem" mammals, whose fossil attributes show a clear transition between advanced cynodonts of the Middle and Upper Triassic, and the earliest mammals. 

        Other stem mammals, described below, include the genera Adelobasileus and Hadrocodium, and the families of morgonucodonts and docodonts, each with several genera. These do not comprise a closely related or monophyletic group, but are resolved into a series of branches or clades ranked in successively closer order to the main, or crown mammal group, in the following order: Adelobasileus, Sinoconodon, morganucodonts, docodontans, and Hadriocodium (Luo et al. 2002; Kielan-Jaworoska et al. 2004). Of these, the inclusion of Adelobasileus, the oldest taxon, is the least secure.

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Adelobasileus

           Fossils of Adelobasileus cromptoni were discovered by Lucas and Hunt (1990) in Texas in the form of a partial cranium or skull, and teeth. The fossils were found near Kalgary in Crosby County, Texas in layers from the Tecovas Member of the Dockum Formation, dated as Late Triassic (Late Carnian phase), about 225 mya.  The Adelobasileus fossils are thus considered at least ten million years older than than those of any previously described, proposed mammal (Lucas and Luo 1993).         

            The skull or cranium of Adelobasileus, about 1.7 cm long, represents a very small animal. While the lower, rear part of the skull (basicranium) shares  numerous traits with early Jurassic  mammals, other cranial features such as the petrosal region, which has only a small or incipient promontorium housing the ear cochlea, indicate only an intermediate stage of the character transformation from non-mammalian cynodonts to  mammals. Since the full development of the mammalian ear is not present,  Adelobasileus has been excluded from the group including Sinocodon and all extant mammals (Lucas and Luo 1993; Luo et al. 2002). The fossil remains of Adelobasileus are presently considered insufficient for any more precise identification (Kielan-Jarowoska et al. 2004).

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Sinoconodonts

Class: Synapsida;  Order: Therapsida;   Suborder:  Cynodontia; clade (unranked): Mammaliamorpha; Family: Sinoconodontidae

            Sinoconodon ("Chinese tooth") was a very ancient proto-mammal (Mammaliaform) from the early Jurassic (208-191 Mya)Sinoconodon, regarded as the most basal of the mammaliaforms, is the sistertaxon to a clade that includes Morganucodon and the living mammals ( Luo et al 2002). 

Discovery:

Sinoconodon rigneyi  (Patterson & Olson, 1961)

        Sinoconodon rigneyi appears in the fossil record of Yunnan province, China in the dark red beds of the Lower Lufeng Series, representing the Sinemurian stage of the Early Jurassic period (193 mya). Its holotype consists of a poorly preserved skull and post-cranial fragments from several individuals.

Anatomy:
           It shows a unique combination of reptilian and mammalian features. It possessed a secondarily evolved jaw joint between the dentary and the squamosal bones, which had replaced the primitive reptilian joint between the articular and quadrate bones. It retained the double jaw joint seen in other advanced Cyndonts. Here, however, while the reptilian jaw joint was still present  (though tiny), the mammalian jaw joint was stronger, with a large dentary condyle fitting into a distinct fossa on the squamosal. This refinement of the jaw joint is the main criterion for identifying Sinoconodon as a Mammaliaform.

        Its eye socket was now fully mammalian, with a closed medial wall. It has an expanded hindbrain, one of the key traits of early mammals. Although the animal appears closely related to more mammalian Morganucodon, it differed substantially from it in in its dental and growth habits. Like mammals, it had permanent cheekteeth or molars. At the same time, like the reptiles, Sinocodon was polyphyodont, replacing many of its teeth throughout its lifetime, and it seems to have grown slowly but continuously until its death.  Even the smallest known individuals had already began the teething cycle in their front teeth.    

            Other fossil species of Sinoconodon include 1) Sinoconodon parringtoni (Young, 1982).   The holotype (IVPP V7203, V4726) is a skull, found in the Lower Lufeng Series of Yunnan, China, dating, like S. rigneyi and other Sinoconodon fossils, from the Sinemurian stage of the Early Jurassic period (193 mya)  2) Sinoconodon changchiawaensis   (Young, 1982), synonomous with  Lufengoconodon changchiawaensis (Young, 1982). From the Lower Lufeng Series in Yunnan; its holotype (IVPP 4727) is a skull.     3) Sinoconodon yangi  (Zhang & Cui, 1983). Also from the Lower Lufeng Series in Yunnan. Holotype: a skull. (A.W. Crompton and A. L. Sun. 1985)

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Morganucodonts  

Class: Synapsida;  Order: Morganucodonta;   Family: Morganucodontae

Discovery:

            Morganucodonts ("Morgan's tooth"), named for a site in Glanmorgan, Wales, includes some of the earliest mammalians from the the Late Triassic to Middle Jurassic (~ 220-180 mya). Three main genera are known, Morganucodon, Eozostrodonand Haldanodon, representing a diverse family with global distribution in the northern hemisphere (Laurasia), as well as Africa and India in the southern, Gondwanan region (Kielan-Jaworoska et al. 2004)

            The species Morganucodon watsoni was discovered and named in 1949 by Walter Georg Kuhne on the basis of a lower molar he found in fissure fillings of Duchy Quarry in Glanmorgan. Thousands more specimens of M. watsoni, including teeth. jaw fragments, and bones, were subsquently found and described by Kuhne in 1958, and by Kermack and others in 1956 and 1958. The fissure fillings at Glanmorgan were dated by Kermack et al. (1981) to be from the Early Jurassic, possibly the Sinemurian phase (200-190 mya).

            Kuhne also found tooth fossils from this period at the Holwell Quarry in England, which in 1941 were placed by Parrington in a related genus named Eozostrodon.  Other Morganucodonts have since been found in Switzerland and France, with additional taxa being named, including Warelolestes and Helvetiodon. These European examples of Morganucodontans show a high level of species diversity (Clemens 1986; Kielan-Jaworoska et al. 2004).

            In China, 1941 explorations in the rich fossil beds of the Lower Lufang formation in Yunnan province revealed a complete skull of Morganucodon. This was first identified by E.T. Oehler in 1948, and later described by W. H. Rigney as Morganucodon oehleri. The two distant species, M. watsoni from Wales and M. oehleri from China, were jointly described by Kermack et al. (1973, 1981). Additional skulls were found in Yunnan in the 1970s and 80s by the Beijing Institute of Vertebrate Paleontology and Paleoanthropology (Young 1982; Zhang 1984; Crompton and Luo 1994).

            In North America, isolated teeth of Morganucodon were found in Early Jurassic deposits in Arizona by Jenkins and his coworkers, along with more abundant teeth and skull fragments, placed in a related genus named Dinnetherium (Jenkins et al. 1983; Crompton and Luo 1993). Other, more isolated examples of Morganucodonts have been found in Greenland (Jenkins et al 1994), and central Russia (Gambarayan and Averianov 2001; Kielan-Jaworoska et al. 2004).

            Four additional taxa of Morganucodonts have been reported from the Early Jurassic region of Gondwana in the southern hemisphere. In southern Africa, two genera were named in the 1960s, Erythrotherium (Crompton 1964), and Megazostrodon (Crompton and Jenkins 1968). More recently, in the 1990s, two taxa have been named for related fossil discoveries in India, Gondwanadon (Datta and Das 1996), and Indozostrodon (Datta and Das 2001; Kielan-Jaworoska et al. 2004).

Anatomy:

            Morganucodonts were small, with skulls 27-38 mm long and body weight estimated at ranging from 27-80 grams (Luo, Crompton, and Sun 2001).  They have many features in their skull representing general mammalian traits, which first appear in  morganocodonts (in cladistic terms, are primitive to them). Like some advanced cynodonts, they have a double or compound jaw joint. This includes the mammalian joint, with the condyle or knob of the dentary bone in articulation with the recess or fossa of the squamosal bone ( the primary diagnostic feature for early mammals). It also includes a retention of the reptilian jaw joint between the articular and quadrate bones. In their skull anatomy, the orbit around the eye (orbitosphenoid region) is fully enclosed, as in all mammals. 

        In their basioccipital region (lower back of the skull), there is a petrosal area with a small mound or promontorium, a trait shared by mammals and some advanced cynodonts. There too, in the area of the ear and petrosal, is a juncture of small postdentary bones. In Morganucodon, these bones, notably, lie enclosed in a postdentary trough, which  is a primitive character of non-mammalian cynodonts (Kermack et al. 1973; Kielan-Jaworoska et al. 2004).  The trough forms a concavity on the side of the mandible, into which fits the angular bone and the malleus, part of the middle ear (Crompton and Luo 1993). 

Fig. 1: Lower jaw of Morganucodon, showing Meckel's groove and the post-dentary trough.

        A diagnostic feature of reptilian jaws called Meckel's groove, which contains Meckel's cartilage, appears in mammals only during early weeks of embryonic growth around the development of the ear region. An attenutated version of Meckel's groove is also present in the adult Morganucodonts, as a primitive feature extending forward from the postdentary trough. These (the postdentary trough, and Meckel's groove) are among the most diagnostic landmarks of the lower jaw bones, which enable the accurate classification of fossils as either early mammals, mammaliformes (called stem mammals),  cynodonts, or other  non-mammals. In the case of morganuconodonts, both Meckel's groove and the angular bone in the trough are more reduced than in Sinoconodon or nonmamalian cynodonts (Luo 1994; Kielan-Jaworoska et al. 2004).

            In many aspects, the teeth of morganucodonts are also distinct from those of Sinoconodon.  Morganucodonts do not have multiple replacement of anterior teeth, a primitive trait found in Sinocodon and synodonts, and today retained by reptiles. Also unlike the latter, the cheek teeth of morganucodonts are divided into triangular-cusped molars and premolars, which have precise occlusion or surface matching in upper and lower teeth (Crompton 1974). All three traits - the lack of multiple tooth replacement, division of cheek teeth in molars and premolars, and precise occlusion of cheek teeth - are all fully mammalian.

             Morganucodon have many mammalian features in their anatomy, and are one of the most widespread of stem mammals. Based on some primitive cranial features, however, such as retention of the double jaw joint, and some postcranial elements, Morganucodonts are not included in the large group of mammals related to today's mammls (called the Crown Mammallia, in cladistic parlance) (Luo et al. 2002).

Docodonts.

    Docodonta is an extinct order of mammals from the Middle Jurassic - Lower Cretaceous in North America, Europe and Asia. A possible example has also been reported from the Upper Cretaceous of Argentina

    Docodonts are characterized by complicated lower molars. They also had the mammalian dentary-squamosal jaw joint.  

Haldanodon  Lillegraven and Krusat 1991

 Relatively well preserved fossils of Haldanodon were found in Upper Jurassic strata at Guimarota, Portugal. These include a number of skulls and one skeleton. The short, broad upper arm bone (humerus) suggests this animal was adapted for digging. Given the swampy conditions which then prevailed at thise location, Haldanodon may well have been a semi-aquatic, riverine animal.

Sibirotherium

The latest known docodont from the northern hemisphere was Sibirotherium from the Lower Cretaceous of Eastern Russia, first described in 2003. 

Boreolestes

A partial skeleton of Boreolestes has been reported, (but not yet described), from the Middle Jurassic of Britain. 

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Hadrocodium 

            Hadrocodium is a tiny, early mammaliaform from the Early Jurassic (195 mya) of China,which had a significantly large brain size, indicated by the wide shape of its skull (Luo 2001). Based on an extensive analysis by Luo et al (2002), Hadrocodium is seen as the sister taxon to the clade of triconodontids and extant Mammalia  Among 15 comparative taxa, it is more closely related to living mammals than are Adelobasileus, Sinoconodon, morganucodontids, and Haldanodon.  

     Hadrocodium wui  (Hadro, "fulness"; -codiium, "head", for its very large brain capacity; wui, after Dr. X-C Wui, discover of the fossil taxon in 1985.) The holotype, a nearly complete skull, was found in the Lower Lufang Formation in Yunnan, China,  dating from the Sinemurian phase of the Early Jurassic. It was originally classified as Morganucodon sp., then renamed Hadrocodium wui by Luo et al (2001).

    Hadrocodium wui was a tiny insectivore with triconodont-like teeth who lived at the beginning of the Jurassic period in a lowland, possibly estuarine environment represented by the Lufang shales of Yunnan . It is considered by Luo et al (2002) to shed significant light on the evolution of the mammalian middle ear. It is the earliest known taxon that lacks the primitive attachment of the middle ear bones to the mandible but has an enlarged brain vault (suggestive of a large brain). 

        Hadrocodium shows a key correlation between a) the separation of the middle ear bones from the mandible and b) the expanded brain vault. Its skull features show that several key mammalian evolutionary innovations in the ear region, the temporo mandibular joint, and the brain vault evolved incrementally through mammaliaform evolution, long before the differentiation of the living mammal groups.

            This extends the first appearance of these modern mammalian features back to the Early Jurassic, some 45 million years earlier than the next oldest mammals that have preserved such derived features, such as Triconodon from the Late Jurassic. All other nonmammalian mammaliaforms with small brain vaults have retained the mandibular attachment of the middle ear bones, whereas Hadrocodium and living mammals with larger brain vaults have lost the mandibular attachment to the middle ear. (Luo et al. 2001)

            As Luo et al (2002) indicate, the cranial vault of Hadrocodium is wider and more expanded in the alisphenoid and parietal region than those of all other nonmammalian mammaliaforms and all other Jurassic mammals so far known. The brain vault in the parietal region in Hadrocodium is comparable to those of Triconodontidae (Simpson 1928), multituberculates (KielanJaworowska 1986, 1997), and the mammalian crown group (KielanJaworowska 1986, 1997; Rowe 1996a, b), but wider than in nonmammaliaform cynodonts (Quiroga 1979), Sinoconodon (Patterson and Olson 1961; Crompton and Sun 1985; Crompton and Luo 1993; Luo 1994), Morganucodon (K.A. Kermack et al. 1981; Rowe 1996a, b), and Haldanodon (Lillegraven and Krusat 1991). On the basis of the allometric scaling of a large sample of living and fossil mammals, Luo et al (2002) determined that the brain vault of Hadrocodium is larger than expected for the mammals of its comparable skull width, and far wider than in any other Triassic-Jurassic mammaliaforms.

            In mammaliforms below the level of Hadrocodium, the occiput, the jaw joint, and the transverse part of the zygomatic arch were all at more or less the same level of the skull. The squamosal portion of the arch flared out widely to accomodate the bulging jaw musculature. However, the muscles driving the jaw still took up a great deal of room.(Palaois). In Hadrocodium, jaw articulation is moved anteriorly, so that the jaw musculature is spread forward as well, leaving more room for the brain cavity in the posterior half of the skull. The reason for this is very likely the separation of of hearing from eating. The jaw was able to move forward only because it was freed from its connection with the otic capsule. (Palaois)

     In Hadrocodium, the petrosal area of the rear, lower skull, enclosing the middle and inner ear, the has a typical mammalian feature, a prominent promontorium (bony housing of the inner ear cochlea). This is more inflated than those of other mammaliaforms, triconodontids, most multituberculates, and nontribosphenic therians. Other derived or non-primitive features (called apomorphies) of the petrosal area include the following: a) The paroccipital region of the petrosal lacks bifurcation of the paroccipital process into anterior and posterior parts. b) A shallow fossa incudis is welldeveloped lateral to the crista parotica. c) The pterygoparoccipital foramen for the superior ramus of the stapedial artery is completely enclosed by the petrosal.  By contrast  Morganucodon, Sinoconodon, and tritylodontids  lack these apomorphies (Palaios; Wible and Hopson 1993; Wible and Hopson 1995; Rougier, Wible, and Hopson 1996), although Adelobasileus may be an exception (Lucas and Luo 1993).

             With a skull length 12 mm, and an estimated body weight of 2 grams, Hadrocodium ranks among the smallest known mammals, and is considered the smallest mammal yet discovered in the Mesozoic. The smallest living insectivoran placental has an adult weight of about 2.5 g. The smallest bat has an adult weight of about 2.0 g . The smallest Cenozoic fossil insectivore mammal has an estimated body weight of 1.3 to 2.04 g (Bloch et al. 1998).

        The very small size of Hadrocodium contrasts to its contemporary mammaliaforms of the Late Triassic and Early Jurassic, such as Sinconodon, whose weight is estimated from 13 to 517 grams, based on skull length from 22 to 62 mm; and Morganucodon, ranging in weight from 27 to 89 grams, based on skull length from 27 to 38 mm). This wide range of body sizes indicates a diversity in the specialization of triconodont-like insectivores in the Lufeng mammaliaform fauna.


 

Crowngroup Mammalia 

        This group is defined by Luo et al. (2002) as the common ancestor of all living mammals and all its descendants. A dental synapomorphy shared by all fossil and living members of this clade is the presence of occlusal surfaces that match precisely between upper and lower molars upon eruption (Rowe 1988; Rowe and Gauthier 1992; McKenna and Bell 1997).

        A mandibular synapomorphy of this grouping is the presence of a distinctive masseteric fossa with a welldefined ventral margin. This fossa occupies the entire mandibular angle region and is far more expanded than in such stem taxa as Sinoconodon, morganucodontids, Kuehneotherium and Hadrocodium.

        The cochlear canal is more elongate (although not always coiled) in all members of the mammalian crown group in which this feature is known (Luo and Ketten 1991; Meng and Wyss 1995; Fox and Meng 1997; Hurum 1998b), at least in comparison to Sinoconodon (Luo et al. 1995), Morganucodon (Graybeal et al. 1989), Haldanodon (Lillegraven and Krusat 1991) and cynodonts (Allin and Hopson 1992; Luo 2001).

        All clades of the mammalian crowngroup lack an ossified pila antotica separating the cavum epiptericum from the braincase, with the notable exception of multituberculates (Hurum 1998a).

        The astragalus and calcaneus are in partial superposition in the majority of the taxa for which the tarsals are known (except Ornithorhynchus; also Jeholodens if the latter proves to lie among crown mammals).

        Most living orders of this group have greatly enlarged gyrencephalic cerebral hemispheres (with external gyri and sulci on the surface of endocast).

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Glossary  

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References:

Bloch, J. I., K. D. Rose, P. D. Gingerich, 1998  J. Mammal. 79, p. 804

Crompton, A.W. and Zhe-Xi Luo 1993

Crompton, A.W.  and A. L. Sun 1985   Cranial structure and relationships of the Liassic mammal Sinoconodon. Zoological Journal of Linnean Society 85, pp. 99-119.

Fairman (1999)

Hu et al. (1997)

Kielan-Jaworowska, Z; Luo, Z-X; Cifelli, RL (2004). Mammals from the Age of Dinosaurs. Columbia University Press. Chapter 4. 

Lucas, S.G. and A. P. Hunt. 1990. "The oldest mammal." New Mexico Journal of Science 30:41-4

Lucas, S.G. and Z. Luo, 1993. "Adelobasileus from the Upper Triassic of West Texas: The Oldest Mammal." Journal of Vertebrate Paleontology, Vol. 13(3) pp. 309-334

Luo, Zhe-Xi 1994

Luo, Zhe-Xi, A.W. Crompton,. and Ai-Lin Sun 2001. Science 292, pp. 1535-1540

Luo, Zhe-Xi; Kielan-Jaworowska, Z; Cifelli, RL (2002). "In quest for a phylogeny of Mesozoic mammals". Acta Palaeontologica  Polonica 47 (1), pp.1–78

McKenna and Bell 1997

Palaios

Patterson, B., and E. C. Olson, 1961. A Triconodontid Mammal from the Triassic of
Yunnan: Internat. Colloq. On the Evolution of Mammals. Kon. Vlaamse Acad. Wetensch. Lett. Sch. Kunsten Belgie, Brussells, part 1, pp. 129-191.

Qiang et al. (1999)


Rougier, Wible, and Hopson 1996
 Rowe 1988

Rowe 1993

Rowe and Gauthier 1992

Wang et al. (2001).

Wible and Hopson 1993

Wikipedia  

Young, C-C. 1982. Two primitive mammals from Lufeng, YunnanSelected Works of Yang Zhungjian. Science Press, Beijing, pp. 21-25.

Zhang, F. and G. Cui, 1983. New material and new understanding of Sinoconodon. Vertebrata Palasialica, 21, pp. 32-41.





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