É possível ter uma restrição exclusiva no tipo de um elemento?

Digamos que eu tenha uma Arca de Noé em que Animal / @ name seja único.

Abaixo está um XML que não valida contra o esquema:

<ns:NoahsArk xmlns:ns="http://www.xxx.com" xmlns:xs="http://www.w3.org/2001/XMLSchema-instance" xs:schemaLocation="http://www.xxx.com Persons.xsd"> 
    <Animal xs:type="ns:Dog" ns:name="Gipsy" ns:pedigree="caniche"/>
    <Animal xs:type="ns:Spider" ns:name="Gipsy" ns:legNumber="5"/>
</ns:NoahsArk>


<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:ns="http://www.xxx.com"
targetNamespace="http://www.xxx.com" elementFormDefault="unqualified" attributeFormDefault="qualified"> 
    <xs:element name="NoahsArk">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="Animal" maxOccurs="unbounded" type="ns:Animal"/>
            </xs:sequence>
        </xs:complexType>
        <xs:unique name="NameUnicity">
            <xs:selector xpath="Animal"/>
            <xs:field xpath="@ns:name"/>
        </xs:unique>
    </xs:element>
    <xs:complexType name="Animal" abstract="true">
        <xs:attribute name="name" type="xs:string" use="required"/>
    </xs:complexType>
    <xs:complexType name="Dog">
        <xs:complexContent>
            <xs:extension base="ns:Animal">
                <xs:attribute name="pedigree" type="xs:string" use="required"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    <xs:complexType name="Spider">
        <xs:complexContent>
            <xs:extension base="ns:Animal">
                <xs:attribute name="legNumber" type="xs:integer" use="required"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>    
</xs:schema>

Isso é bom, mas agora vamos dizer que eu quero uma Arca de Noé onde Animal / @ xsi: type seja único.

Eu tentei esta restrição:

<xs:unique name="AnimalUnicity">
    <xs:selector xpath="Animal"/>
    <xs:field xpath="@xs:type"/>
</xs:unique>

Mas este XML ainda é válido :(

<?xml version="1.0" encoding="UTF-8"?>
<ns:NoahsArk xmlns:ns="http://www.xxx.com" xmlns:xs="http://www.w3.org/2001/XMLSchema-instance" xs:schemaLocation="http://www.xxx.com Persons.xsd">
    <Animal xs:type="ns:Dog" ns:name="Pierre-Louis" ns:pedigree="doberman"/>
    <Animal xs:type="ns:Spider" ns:name="Gipsy" ns:legNumber="5"/>
</ns:NoahsArk>

Alguma ideia ?

Obrigado, -E